GRB 260310A
GCN Circular 45175
Subject
GRB 260310A / AT2026fgk: Late-time Chandra Detection
Date
2026-07-17T16:44:21Z (2 months ago)
From
Rahul Jayaraman at Cornell University <rj438@cornell.edu>
Via
Web form
Rahul Jayaraman (Cornell), K-Ryan Hinds (Caltech), Daniel Perley (LJMU), Anna Y. Q. Ho (Cornell), Gilad Sadeh (MPI) report:
The Chandra X-ray Observatory observed the field of GRB 260310A (Fermi GBM team, GCNs 43951, 43975; AstroSatCZTI Collaboration, GCN 43958) in response to a Director’s Discretionary Time request (PI: Jayaraman). The observations, using the ACIS-S instrument, lasted for 8 ks, beginning at 2026-07-15 05:29:10 (UTC; 127 days post-burst).
A source is detected at the position of AT2026fgk, the multi-wavelength transient associated with the GRB (Konno et al., GCN 43974; Jayaraman et al., GCN 43994; Rhodes et al., GCN 44005), at a significance of 6.2-sigma (calculated using `wavdetect`). We detect 14 net counts from this source. A preliminary analysis suggests that this corresponds to an unabsorbed flux of roughly (3 ± 0.9) x 10^-14 erg/s/cm^2, which is broadly consistent with the decay slope of –2.7 measured from the two previous Chandra observations, at 40 and 68 days post-burst (reported in Yang et al., GCN 44375; Yang et al., arXiv:2605.23818; Hinds et al., arXiv:2606.05146).
More detailed analysis is ongoing. We thank Pat Slane and the Chandra Director’s Office for rapidly approving and executing these observations.
GCN Circular 44462
Subject
GRB 260310A / AT2026fgk: photometric monitoring and tentative maximum of a possible SN
Date
2026-05-04T19:46:05Z (4 months ago)
From
Alina Volnova at IKI RAS <alinusss@gmail.com>
Via
Web form
N. Pankov (IKI), A. Pozanenko (IKI), A. Volnova (IKI), E. Schekotikhin (IKI), E. Klunko (ISTP), M. Eselevich (ISTP), A. Moskvitin (SAO RAS), O. Spiridonova (SAO RAS), V. Rumyantsev (CrAO), A. Novichonok (KIAM), A. Kochergin (UAFO IAA), I. Sokolov (INASAN), M. Krugov (FAI), R. Inasaridze (AbAO), A. Tarasenkov (INASAN) report on behalf of IKI GRB-FuN:
We observed the optical counterpart AT2026fgk (a.k.a. GOTO26buh, O'Neill et al. 2026, TNS Discovery Report 294132; Hinds et al., AstroNote 2026-65; Konno et al., GCN 43974) associated with GRB 260310A (The Fermi GBM team, GCN 43951; Salunke et la., GCN 43958; Hamburg & Meegan, GCN 43975; Jayaraman et al., GCN 43994). Observations began on March 13, 2026, and continue to this day. Observations were performed with the following telescopes: AZT-33IK (Mondy), Zeiss-1000 (SAO RAS), Zeiss-1000 (Koshka), ZTSh (CrAO), ORI-50 (UAFO), Zeiss-2000 (Terskol), AZT-20 (Assy), AS-32 (AbAO) with BR-filters, and BVRI-filters on a few epochs.
We clearly detect the counterpart (Lipunov et al., GCN 43954, 43978; Martin-Carrillo et al., GCN 43979, 44000; Moreno Méndez et al., GCN 43979; Hsu et al., GCN 43986; Pursiainen et al., GCN 43990; Becerra et al., GCN 43991, 44059; Jin-Ji Li et al., GCN 43993; Stein et al., GCN 43996; Watson et al., GCN 44001; Lai et al., GCN 44002; Izzo, GCN 44003; Brivio et al., GCN 44004; Moskvitin et al., GCN 44006; Romanov, GCN 44020; Brosio et al., GCN 44021, 44067; Pawar et al., GCN 44022; Belkin & Shrestha, GCN 44043; H. L. Li et al., GCN 44044; Gupta et al., GCN 44051; Passaleva et al., GCN 44058, 44104; Volnova et al., GCN 44060; Busmann et al., GCN 44061; Zheng & Filippenko, GCN 44065; Pankov et al., GCN 44073; Serrau et al., GCN 44112; Kang et al., GCN 44119; Effress et al., GCN 44122; Guelfand et al., GCN 44125; O'Connor et al., GCN 44137; Liu et al., GCN 44172; Freeberg et al., GCN 44208). We confirm the break reported by Perly at al. (GCN 44096) on the 8th day after the trigger.
After subtracting the image using the template obtained on April, 21 in the R filter with AZT-33IK, we constructed the afterglow + supernova light curve (LC). We note, that the point source is still clearly detected on the image on May, 02.
We note that the afterglow power law decay index of -2.1 is steeper than reported by Perly at al. (GCN 44096) and may be affected by the still glowing optical counterpart.
Subtracting the power-law afterglow with the power law decay index of -2.1 from the light curve gives us a possible supernova light curve, shown in
https://heaiki.ru/lvc/GRB260310A/GRB260310A_subtracred_host_and_PL__afterglow.jpg
(for the LC construction we use only observations obtained with AZT-033IK, Mondy.)
Thus we can tentatively confirm the maximum of bump which was suggested as possible SN associated with GRB 260310A in R-filter of about 21 days after the GRB trigger reported earlier (Freeberg et al., GCN 44248), however more faint and with apparent magnitude of about R ~ 21.8. Thus absolute magnitude at the redshift of z=0.153 (Hinds et al., GCN 43977; de Ugarte Postigo et al., GCN 43984) would be not fainter than M_R = -17.5. Initially SN rising was suggested by SED evolution (de Ugarte Postigo et al., GCN 44124).
We continue to monitor the optical counterpart of the GRB 260310A.
GCN Circular 44375
Subject
GRB 260310A: Late-time Chandra X-ray Detection
Date
2026-04-21T10:40:36Z (5 months ago)
From
Yu-Han Yang at University of Rome Tor Vergata <yyang@roma2.infn.it>
Via
Web form
Yu-Han Yang (U Rome), Eleonora Troja (U Rome), Yi-Han Iris Yin (HKU), Brendan O’Connor (CMU), Muskan Yadav (U Rome), Niccolo’ Passaleva (U Rome), Roberto Ricci (U Rome) report:
We imaged the field of GRB 260310A (Fermi GBM team, GCN 43951,43975; AstroSat CZTI collaboration, GCN 43958) with the Chandra X-ray Observatory via a Director’s Discretionary Time (DDT) request (PI: Yang). Observations started at 2026-04-20T01:47:42 (~41 days after the trigger) using the ACIS-S instrument with a total exposure time of ~5 ks.
Three sources are detected within a 1-arcmin radius centered on the optical counterpart (O’Neill et al. 2026, TNS Discovery Report 294132). The target afterglow is clearly identified with 89 counts, while two neighboring sources are marginal contributors, yielding only 5 and 9 counts, respectively. We performed a preliminary spectral analysis of the afterglow in the 0.5–8 keV range, adopting an absorbed power-law model with a fixed galactic absorption (N_H = 2.59e20/cm2) and intrinsic absorption (N_Hz = 1.7e21/cm2). We find a photon index of 2.07 ± 0.25 and a corresponding unabsorbed flux of (5.9+/-0.8)e-13 ergs/cm2/s (0.3-10 keV).
Our observations confirm that the X-ray rebrightening (Jayaraman et al., GCN 44234; Waratkar et al.; GCN 44278) is intrinsic to the GRB afterglow and not contributed by nearby sources. Our temporal analysis indicates that the light curve is consistent with a late-time bump which is currently transitioning into a decay phase.
We thank the Chandra director, Pat Slane, for awarding discretionary time and the observatory staff for rapidly scheduling these observations.
GCN Circular 44278
Subject
GRB 260310A / AT2026fgk: NuSTAR detection of the rebrightening
Date
2026-04-12T20:47:14Z (5 months ago)
From
Gaurav Waratkar at Caltech <gauravwaratkar@iitb.ac.in>
Via
Web form
Gaurav Waratkar (Caltech), Elias Kammoun (Caltech), Rahul Jayaraman (Cornell), K-Ryan Hinds (Caltech), Anna Y. Q. Ho (Cornell) report:
In response to our NuSTAR DDT request (PI Waratkar), NuSTAR observed GRB 260310A (Fermi GBM team, GCN 43951; Salunkhe et al., GCN 43958), associated with the afterglow AT2026fgk (Hinds et al., TNS AstroNote 2026-65; Konno et al., GCN 43974) beginning at 2026-04-11T13:45:07 UTC, about 32 days after the GRB.
A total of 12.7 ks of observations were obtained from both the FPMA and FPMB modules. We clearly detect the X-ray afterglow at the location of AT2026fgk. We performed a preliminary spectral analysis in the 3–79 keV energy range, fitting an absorbed power law. We find a photon index of 1.64 ± 0.25 with an unabsorbed flux of (4.49 ± 0.67) x 10^-13 ergs/cm2/s (2-10 keV). All uncertainties are reported at the 90% CL.
Our detection agrees with the deviation from the post-break afterglow decay reported in EP/FXT observations (Jayaraman et al., GCN 44234).
We thank the entire NuSTAR Science & Mission operations teams for the rapid approval and execution of this DDT observation. NuSTAR is a NASA Small Explorer mission led by Caltech and managed by JPL for NASA's Science Mission Directorate in Washington.
GCN Circular 44248
Subject
GRB 260310A/AT2026fgk: Kilonova-Catcher SN peak identification
Date
2026-04-09T13:57:11Z (5 months ago)
From
Damien Turpin at CEA-Saclay <dturpin-astro@hotmail.com>
Via
Web form
M. Freeberg, M. Serrau (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), S. Antier (IJCLab), M. Coughlin (UMN),S. Karpov (FZU), P. Hello (IJCLAB), M. Pillas (IAP) on behalf of the GRANDMA/Kilonova-Catcher collaboration:
We have continued to observe the field of GRB 260310A detected by Fermi/GBM (Fermi GBM team, GCN 43951) and AstroSat CZTI (Salunke et al., GCN 43958) with the GRANDMA citizen science project Kilonova-catcher (KNC) over the past days mainly in Rc and sdssr filters. Our last night observations were performed with a TEC180FL telescope operated by M. Freeberg. Our observations started at TGRB+29.9 days and were taken with Rc filters.
In our stacked frames, subtracted from the PanSTARRS DR2 template image, we still detect the optical counterpart at Rc = 20.39+/-0.18 at a midtime T-TGRB = 29.97 days. Despite the moderate Signal-to-Noise ratio (SNR) in our image series, we tentatively identified the supernova peak brightness at Rc = 19.9+/-0.1 around T-TGRB = 21.8 days.
At z = 0.153 (de Ugarte Postigo et al., GCN 43984), such a brightness and peak time seem in good agreement with the expected peak brightness and time of the redshifted canonical GRB/SNe SN 1998bw.
The KNC members will keep monitoring the source as long as it stays detectable.
All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained with the Johnson-Cousins filters were calibrated using the GAIA DR3 Synphot catalog.
We use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023).
GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/).
GCN Circular 44235
Subject
GRB 260310A: Continued VLA Multi-frequency Radio Observations
Date
2026-04-08T02:27:55Z (5 months ago)
From
Genevieve Schroeder at Cornell University <genevieveschroeder@u.northwestern.edu>
Via
Web form
G. Schroeder (Cornell), D. A. Perley (LJMU), and T. Laskar (Utah) report on behalf of a larger collaboration:
The Karl G. Jansky Very Large Array observed the location of the afterglow (Hinds et al., TNS AstroNote 2026-65; Konno et al., GCN 43974) associated with GRB 260310A (Fermi GBM Team, GCN 43951; Hamburg & Meegan, GCN 43975) using all available receivers from L-band to Q-band, providing nearly continuous spectral coverage from 1 to 50 GHz, as a part of our multi-frequency public campaign (Perley et al., GCN 44160). Observations were carried out on UT 2026-04-04 between 08:42 and 12:19 UT, approximately 25.2 days after the GRB.
Preliminary flux densities are:
| freq(GHz) | flux(mJy) |
|---|---|
| 1.5 | 1.74 +/- 0.06 |
| 3 | 5.00 +/- 0.04 |
| 6 | 8.39 +/- 0.05 |
| 10 | 8.93 +/- 0.12 |
| 15 | 8.16 +/- 0.22 |
| 22 | 7.34 +/- 0.23 |
| 33 | 6.14 +/- 0.19 |
| 45 | 5.25 +/- 0.19 |
Compared to previous radio observations (Rhodes et al., GCN 44005; Giarratana et al., GCN 44045; Ho et al., GCN 44057; Ho et al., GCN 44134, Perley et al., GCN 44160), the afterglow is brightening at lower frequencies (< 10 GHz) and fading at higher frequencies (> 10 GHz).
We thank the NRAO staff for scheduling and executing these observations.
Further observations are planned.
GCN Circular 44234
Subject
GRB 260310A / AT2026fgk: X-ray rebrightening
Date
2026-04-07T17:22:52Z (5 months ago)
From
Rahul Jayaraman at Cornell University <rj438@cornell.edu>
Via
Web form
Rahul Jayaraman (Cornell), K-Ryan Hinds (Caltech), Gaurav Waratkar (Caltech), S. Y. Fu (HUST), R. D. Liang, M. J. Liu, Z. X. Ling, Hui Sun, Weimin Yuan (NAO, CAS), Anna Y. Q. Ho (Cornell), Genevieve Schroeder (Cornell) report:
The Follow-up X-ray Telescope (FXT) on board the Einstein Probe has continued to monitor the X-ray afterglow of GRB 260310A (Fermi GBM team, GCN 43951; Arya et al., GCN 43958; Jayaraman et al., GCN 43994), associated with the optical transient AT2026fgk (Konno et al., GCN 43974; Hinds et al., AstroNote 2026-65), with 10 epochs of observations since the GRB.
After the detection of a clear break (near-simultaneous steepening in both the X-ray and optical; Jayaraman et al., GCN 44095; Perley et al., GCN 44096), we observed two further epochs that were consistent with the steeper slope of -1.6 reported in GCN 44095. However, the most recent three epochs (which span 3 days) show a clear rebrightening, with a power-law index of α = 2.9 ± 0.5, where f ~ t^α.
Further follow-up with FXT is planned.
Launched on January 9, 2024, the Einstein Probe is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is supported by the Strategic Priority Program on Space Sciénce of the Chinese Academy of Sciences, in collaboration with the European Space Agency, the Max-Planck-Institute for Extraterrestrial Physics (Germany), and the Centre National d'études Spatiales (France).
GCN Circular 44208
Subject
GRB 260310A/AT2026fgk: Kilonova-Catcher long term optical monitoring
Date
2026-04-03T13:30:44Z (5 months ago)
From
Damien Turpin at CEA-Saclay <dturpin-astro@hotmail.com>
Via
Web form
M. Freeberg (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), S. Antier (IJCLab), M. Coughlin (UMN),S. Karpov (FZU), P. Hello (IJCLAB), M. Pillas (IAP) on behalf of the GRANDMA/Kilonova-Catcher collaboration:
We have continued to observe the field of GRB 260310A detected by Fermi/GBM (Fermi GBM team, GCN 43951) and AstroSat CZTI (Salunke et al., GCN 43958) with the GRANDMA citizen science project Kilonova-catcher (KNC) over the past week mainly in Rc and sdssr filters (One observation every night). Our last night observations were performed with a CDK17 telescope operated by M. Freeberg. Our observations started at TGRB+23.9 days and were taken with Rc filters.
In our stacked frames, subtracted from the Legacy Survey DR10 template image, we still detect the optical counterpart reported by many teams (O'Neill et al. 2026, TNS Discovery Report 294132; Konno et al., GCN 43974; Hinds et al., GCN 43977; Lipunov et al., GCN 43978;Martin-Carrillo et al., GCN 43979; Méndez et al., GCN 43980;de Ugarte Postigo et al., GCN 43984; Hsu et al., GCN 43986; Pursiainen et al., GCN 43990; Becerra et al., GCN 43991; Li et al., GCN 43993; Jayaraman et al., GCN 43994; Stein et al., GCN 43996;Martin-Carrillo et al., GCN 44000; Moreno Méndez et al., GCN 43980; Becerra et al., GCN 43991; Li et al., GCN 43993; Stein et al., GCN 43996; Watson et al., GCN 44001; Lai et al., GCN 44002; Izzo et al., GCN 44003; Brivio et al., GCN 44004; Moskvitin et al., GCN 44006; Romanov, GCN 44020; Brosio et al., GCN 44021; Pawar et al., GCN 44022; Belkin et al., GCN 44043; Li et al., GCN 44044; Gupta et al., GCN 44051; Becerra et al., GCN 44059; Volnova et al., GCN 44060; Busmann et al., GCN 44061; Zheng et al., GCN 44065; Brosio et al., GCN 44067; Pankov et al. GCN 44073; Perley et al. GCN 44096; Passaleva et al. GCN 44104; Kang et al;, GCN 44119; Effress et al., GCN 44122; de Ugarte Postigo et al., GCN 44124; Guelfand et al. GCN 44125; O'Connor et al., GCN 44137; Liu et al., GCN 44172)
We report our last 3 nights follow-up results in the table below:
+------------------+-----------+-----------+----------------+--------------+
| Tmid-TGRB (days) | Exp (s) | Filter | Magnitude | Instrument |
+==================+===========+===========+================+==============+
| 21.82 | 44 x 180s | Rc (Vega) | 19.92 +/- 0.12 | CDK17 |
| 22.82 | 50 x 180s | Rc (Vega) | 20.01 +/- 0.15 | CDK17 |
| 23.95 | 50 x 180s | Rc (Vega) | 20.22 +/- 0.12 | CDK17 |
+------------------+-----------+-----------+----------------+--------------+
The KNC members will keep monitoring the source as long as it stays detectable.
All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained with the Johnson-Cousins filters were calibrated using the GAIA DR3 Synphot catalog.
We use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023).
GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/).
GCN Circular 44172
Subject
GRB 260310A / AT2026fgk: Luminance-band optical follow-up with a 28cm telescope in Daocheng
Date
2026-04-01T07:07:05Z (6 months ago)
From
Y.C. Kang at Peking University <yckang@stu.pku.edu.cn>
Via
Web form
Zhuokai Liu, Zexuan Wu, Chenxi Bao, Yacheng Kang (PKU) report on behalf of the Interestar Collaboration and the PKU HiTF (High-energy Transients Follow-up) group:
We observed the field of GRB 260310A, detected by Fermi/GBM (GCN 43951; T0 = 2026-03-10T04:57:10.81) and AstroSat/CZTI (Salunke et al., GCN 43958), using a 28cm optical telescope located in Daocheng, Sichuan, China. Observations began on 2026 Mar 17, approximately ~ 7.4 days after the GRB trigger, and spanned six epochs in the Luminance filter.
In the stacked Luminance-band images, we detect the optical counterpart at a position consistent with AT2026fgk, the reported GRB afterglow candidate at z = 0.153 (O'Neill et al. 2026, TNS Discovery Report 294132; Hinds et al., AstroNote 2026-65; Konno et al., GCN 43974; Hinds et al., GCN 43977), later found to show Type Ic-BL SN features (de Ugarte Postigo et al., GCN 44124; Guelfand et al., GCN 44125; O'Connor et al., GCN 44137). Preliminary photometry was calibrated using nearby catalogued field stars and is reported as approximate AB-equivalent magnitudes, without correction for Galactic extinction.
Our measurements show that the optical counterpart continues to fade, from ~ 19.0 mag to ~ 19.8 mag between 7.4 and 14.3 days after the GRB trigger. The observation log is summarized below:
| Date | Start_UT | T_mid - T0 (days) | Filter | Exposure (s) | Magnitude (AB) |
|---|---|---|---|---|---|
| 2026-03-17 | 13:28:40 | 7.40 | Luminance | 7400 | 19.02 +/- 0.04 |
| 2026-03-18 | 13:52:51 | 8.42 | Luminance | 7600 | 19.10 +/- 0.04 |
| 2026-03-19 | 14:14:50 | 9.42 | Luminance | 6000 | 19.18 +/- 0.04 |
| 2026-03-20 | 12:37:02 | 10.36 | Luminance | 7400 | 19.39 +/- 0.04 |
| 2026-03-21 | 12:39:41 | 11.35 | Luminance | 5000 | 19.65 +/- 0.05 |
| 2026-03-24 | 12:33:31 | 14.33 | Luminance | 2200 | 19.84 +/- 0.08 |
Further analysis and follow-up observations are ongoing.
The 28cm optical telescope is located in Daocheng, Sichuan, China, and is operated by the Interestar Collaboration. The PKU HiTF group is dedicated to rapid follow-up observations of high-energy transients.
GCN Circular 44160
Subject
GRB 260310A: VLA Multi-frequency Radio Observations
Date
2026-03-30T20:08:21Z (6 months ago)
From
Daniel Perley at Liverpool JMU <d.a.perley@ljmu.ac.uk>
Via
Web form
D. A. Perley (LJMU), G. Schroeder (Cornell), and T. Laskar (Utah) report on behalf of a larger collaboration:
The Karl G. Jansky Very Large Array observed the location of the afterglow (Hinds et al., TNS AstroNote 2026-65; Konno et al., GCN 43974) associated with GRB 260310A (Fermi GBM Team, GCN 43951; Hamburg & Meegan, GCN 43975) using all available receivers from S-band to Q-band, providing nearly continuous spectral coverage from 2 to 50 GHz. Observations were carried out on UT 2026-03-27 between 07:45 and 11:06 UT, approximately 17.2 days after the GRB.
Consistent with previous reports (Rhodes et al., GCN 44005; Giarratana et al., GCN 44045; Ho et al., GCN 44057; Ho et al., GCN 44134) we detect very strong radio emission from the afterglow. Preliminary flux densities are:
| freq(GHz) | flux(mJy) |
|---|---|
| 3 | 2.90 +/- 0.04 |
| 6 | 7.52 +/- 0.05 |
| 10 | 11.11 +/- 0.11 |
| 15 | 11.73 +/- 0.16 |
| 33 | 9.58 +/- 0.39 |
| 45 | 7.98 +/- 0.27 |
This indicates that the afterglow has brightened at low frequencies since the observations of Giarratana et al. and Rhodes et al. at ~4 days post-GRB, and that the peak of the f_nu spectral energy distribution during our observations was at approximately 15 GHz.
We thank the NRAO for rapidly approving our DDT request for a public multifrequency campaign on this exceptional GRB, and Heidi Medlin for assistance with checking the SBs.
Further observations are planned.
GCN Circular 44137
Subject
GRB 260310A: Gemini-North Spectral Confirmation of Supernova Features
Date
2026-03-28T14:25:31Z (6 months ago)
From
Brendan O'Connor at Carnegie Mellon University <boconno2@andrew.cmu.edu>
Via
Web form
B. O'Connor (CMU), S. Toshikage (Tohoku U.), M. Tanaka (Tohoku U.), X. J. Hall (CMU), D. Gruen (LMU), A. Palmese (CMU) report on behalf of a larger collaboration:
We observed the optical counterpart (O’Neill et al. 2026, TNS Discovery Report 294132; Hinds et al., AstroNote 2026-65) of GRB 260310A (Hamburg & Meegan, GCN 43975; Salunke et al., GCN 43958) with GMOS mounted on Gemini-North starting on 2026-03-27 at 14:56:28 UT, corresponding to 17.4 d after the trigger. We obtained a spectral sequence of 2x1060 s exposures using the B480 grating. The observed spectrum has undergone a significant change compared to earlier epochs, and now displays the clear onset of supernova features. We confirm the report of de Ugarte Postigo et al. (GCN 44124) and the association of GRB 260310A to a massive star progenitor.
We acknowledge the excellent support from the staff of the Gemini-North telescope, including Siyi Xu, Atsuko Nitta, Teo Mocnik, Jen Andrews, Sunny Stewart, and Denise Hung.
GCN Circular 44134
Subject
SMA Detection of GRB 260310A / AT 2026fgk
Date
2026-03-27T23:09:57Z (6 months ago)
From
Anna Y Q Ho at Cornell University <ayh24@cornell.edu>
Via
Web form
Anna Ho (Cornell), Garrett Keating (CfA), Kate Alexander (University of Arizona), Tanmoy Laskar (University of Utah), Raffaella Margutti (UC Berkeley) report on behalf of the POETS team:
On UT 2026 March 26 the Submillimeter Array (SMA) observed the position of GRB 260310A / AT2026fgk (GCN 43951, 43958, 43974), a nearby (z=0.153; GCN 43977) gamma-ray burst with bright radio emission at cm (GCN 44005, 44045) and mm (GCN 44057) wavelengths. Observations were obtained as part of the SMA Large Program, POETS (Pursuit of Extragalactic Transients with the SMA; PI Berger). Using the COMPASS pipeline we measure a preliminary flux density of 5.67 +/- 0.23 mJy at 230 GHz.
Further observations are planned. We thank the SMA staff for rapidly approving and scheduling these observations.
GCN Circular 44125
Subject
GRB 260310A / AT 2026fgk: COLIBRÍ photometric evidence of the emerging supernova
Date
2026-03-27T08:14:57Z (6 months ago)
From
enriquemm@astro.unam.mx
Via
Web form
Marion Guelfand (CPPM), Enrique Moreno Méndez (UNAM), Rosa L. Becerra (UNAM), Antonio de Ugarte Postigo (LAM), Noémie Globus (UNAM), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), and Benjamin Schneider (LAM) and report:
We imaged the field of the optical counterpart AT 2026fgk (O’Neill et al. 2026, TNS Discovery Report 294132; Hinds et al., AstroNote 2026-65) of GRB 260310A (Hamburg & Meegan, GCN 43975; Salunke et al., GCN 43958) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-03-27 02:46 to 03:59 UTC (from 16.90 to 16.95 days after the trigger) and obtained 1080 seconds in g, r, and i filters, and 3240 seconds of exposure in the z filter.
The data were reduced, coadded and analysed with the COLIBRÍ ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, it is in the AB system, and it is not corrected for Galactic extinction.
We have obtained spectral energy distributions at multiple epochs prior to T+12 days, all of which are well described by a single power law with a spectral index β ≈ −1. However, tonight we observe a significant increase in the g−r and g-i colours and a decrease in the i−z colour indicating the emergence of a thermal component in the spectral energy distribution. This behaviour suggests the onset of a supernova, in agreement with the report by de Ugarte Postigo et al. (GCN Circ. 44124)
Further observations and analysis are ongoing.
We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM.
COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.
GCN Circular 44124
Subject
GRB 260310A: OSIRIS+/GTC spectroscopic detection of the associated BL-Ic, SN 2026fgk
Date
2026-03-27T07:39:02Z (6 months ago)
From
Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com>
Via
email
A. de Ugarte Postigo (LAM), S. Geier (GTC), L. Izzo (INAF/OACN and DARK/NBI), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), C. C. Thoene (AbAO), M. A. Aloy (UV), J. P. U. Fynbo (DAWN/NBI), L. Galbany (IEEC-CSIC), G. Lombardi (GTC), N. A. Rakotondrainibe (LAM), B. Schneider (LAM), N. R. Tanvir (Univ. Leicester), D. González González (GTC) report:
We observed the optical counterpart (AT2026fgk, a.k.a. GOTO26buh, O’Neill et al. 2026, TNS Discovery Report 294132; Hinds et al., AstroNote 2026-65) of GRB 260310A (Hamburg & Meegan, GCN 43975; Salunke et al., GCN 43958) using the Gran Telescopio Canarias (GTC) equipped with the OSIRIS+ instrument.
A total of 3 spectra of 900 s were secured, starting on 2026-03-27 at 04:25:40 UT (16.98 days after trigger), using grism R1000B. Continuum is visible over the full range 3610-7880 AA. The spectrum has evolved from the power-law continuum reported in de Ugarte Postigo et al. (GCN 43984) and now shows undulations expected in a SN spectrum. Indeed, using SNID to classify the SN we get a best match to a type Ic-BL SN close to maximum light.
Our results allow us to conclusively identify the presence of a SN, linking the event to a massive stellar collapse.
GCN Circular 44122
Subject
GRB 260310A / AT2026fgk: Late-time infrared detection with MIRAGE on the MDM 1.3m telescope
Date
2026-03-27T00:39:12Z (6 months ago)
From
Kishalay De at Columbia University <kde1@mit.edu>
Via
Web form
R. B. Effress, B. T. Gebreamlak, B. O. Goldman, T. Goodman, J. Kalish, L. Koo, T. Lack, S. A. Lawsky, S. L. Manyan, M. Nebart, K. N. Oqueli-White, A. Ramadorai, N. A. Rodgers, T. A. Sabin, E. Sharman, C. Wu, L. Zheng, R. Zheng, G. Mo, V. Karambelkar, S. Ibrahim, N. Aftab, D. Schiminovich, K. De (Columbia University)
We observed the field of GRB 260310A detected by Fermi/GBM (Fermi GB team GCN 43951) and AstroSat CZTI (Salunke et al. GCN 43958), during commissioning operations of the MDM InfraRed Astronomy InGaAs Explorer (MIRAGE) instrument on the MDM 1.3m telescope. Observations were carried out starting at UTC 2026-03-22 08:50, approximately three weeks after the GRB trigger.
We obtained a series of dithered exposures in J-band and Hs-band amounting to a total exposure time of 600 s each. Images were aligned, stacked and calibrated to the 2MASS catalog. Performing forced photometry at the position of the optical counterpart (AT2026fgk; Konno et al. GCN 43974, Hinds et al. AstroNote 2026-65; O’Neill et al. TNS Discovery Report 294132), we clearly detect a point source with magnitudes
J = 18.40 +/- 0.14 Vega mag
Hs = 17.07 +/- 0.16 Vega mag
The measurements confirm continued infrared fading of the source (Gao et al. GCN 44110). Note that these measurements did not include image subtraction, though the host contamination is expected to be small.
Further observations are planned and analysis is being improved. MIRAGE is a new YJH-band near-infrared imager for the MDM 1.3m telescope. We thank the MDM Observatory staff for supporting the commissioning observations.
GCN Circular 44119
Subject
GRB 260310A / AT2026fgk: R- and I-band observations with the PKU 60 cm optical telescope
Date
2026-03-26T13:15:35Z (6 months ago)
From
Y.C. Kang at Peking University <yckang@stu.pku.edu.cn>
Via
Web form
Yacheng Kang, Qiang Wang, Ruize Shi, Zexuan Wu, Zhuokai Liu, Yiming Dong, Ziming Wang, Chenxi Bao, Fangzhou Ren, Lijing Shao, Zhuo Li, Subo Dong (PKU) report on behalf of the PKU HiTF (High-energy Transients Follow-up) group:
The field of GRB 260310A (Fermi GBM Team, GCN 43951; Salunke et al., GCN 43958) was observed with the PKU 60 cm optical telescope at Xinglong Observatory, NAOC. Observations began on 2026 Mar 18, corresponding to ~8.6 days after the GRB trigger, and were conducted over a total of three epochs.
In the stacked R- and I-band images, we clearly detect the optical counterpart AT2026fgk (e.g., Konno et al., GCN 43974; Hinds et al., GCN 43977; Lipunov et al., GCN 43978; Martin-Carrillo et al., GCN 43979; Moreno Méndez et al., GCN 43980; de Ugarte Postigo et al., GCN 43984; Hsu et al., GCN 43986; Malesani et al., GCN 43990; Becerra et al., GCN 43991; Li et al., GCN 43993; Stein et al., GCN 43996; Martin-Carrillo et al., GCN 44000; Watson et al., GCN 44001; Lai et al., GCN 44002; Izzo et al., GCN 44003; Brivio et al., GCN 44004; Moskvitin et al., GCN 44006; Romanov, GCN 44020; Brosio et al., GCN 44021; Pawar et al., GCN 44022; Belkin et al., GCN 44043; Li et al., GCN 44044; Gupta et al., GCN 44051; Passaleva et al., GCN 44058; Becerra et al., GCN 44059; Volnova et al., GCN 44060; Busmann et al., GCN 44061; Zheng et al., GCN 44065; Brosio et al., GCN 44067; Pankov et al. GCN 44073; Perley et al. GCN 44096; Passaleva et al. GCN 44104; Serrau et al. GCN 44112; O'Neill et al. 2026, TNS Discovery Report 294132). Preliminary photometry yields R-band magnitudes ranging from ~18.7 to ~18.9 mag and I ~ 18.5 mag, calibrated against nearby Pan-STARRS field stars and reported in the Vega system, without correction for Galactic extinction.
The observation log is summarized below (T0 = 2026-03-10T04:57:10.81; GRB 260310A):
| Date | Start_UT | T_mid − T0 (days) | Filter | Exposure (s) | Magnitude (Vega) |
| :--------: | :---------: | :---------------: | :----: | :----------: | :--------------: |
| 2026-03-18 | 20:23:26.87 | 8.66 | R | 4 * 600 | 18.66 +/- 0.03 |
| 2026-03-19 | 16:22:31.97 | 9.53 | I | 15 * 600 | 18.45 +/- 0.03 |
| 2026-03-19 | 18:57:02.55 | 9.63 | R | 14 * 600 | 18.90 +/- 0.03 |
Further analysis and follow-up observations are ongoing.
The 60 cm telescope is operated by the Department of Astronomy, School of Physics, Peking University. The PKU HiTF group is dedicated to rapid follow-up observations of high-energy transients.
GCN Circular 44112
Subject
GRB 260310A/AT2026fgk: Kilonova-Catcher late optical afterglow detection at about 15 days post trigger time
Date
2026-03-25T14:03:49Z (6 months ago)
From
Damien Turpin at CEA-Saclay <dturpin-astro@hotmail.com>
Via
Web form
M. Serrau, M. Freeberg (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), S. Antier (OCA/IJCLab), M. Coughlin (UMN),S. Karpov (FZU), P. Hello (IJCLAB), M. Pillas (IAP) on behalf of the GRANDMA/Kilonova-Catcher collaboration:
We observed the field of GRB 260310 detected by Fermi/GBM (Fermi GBM team, GCN 43951) and AstroSat CZTI (Salunke et al., GCN 43958) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with the ODK16 telescope at Observatoire Banon La Tuilerie, France operated by M. Serrau and the TEC160FL telescope operated by M. Freeberg. Our observations started at TGRB+14.8 days and were taken with sdss-g and sdss-r filters.
In our stacked frames, subtracted from the Legacy Survey DR10 template image, we detect the optical counterpart reported by GOTO (O'Neill et al. 2026, TNS Discovery Report 294132) and observed earlier (Konno et al., GCN 43974; Hinds et al., GCN 43977; Lipunov et al., GCN 43978;Martin-Carrillo et al., GCN 43979; Méndez et al., GCN 43980;de Ugarte Postigo et al., GCN 43984; Hsu et al., GCN 43986; Pursiainen et al., GCN 43990; Becerra et al., GCN 43991; Li et al., GCN 43993; Jayaraman et al., GCN 43994; Stein et al., GCN 43996;Martin-Carrillo et al., GCN 44000; Moreno Méndez et al., GCN 43980; Becerra et al., GCN 43991; Li et al., GCN 43993; Stein et al., GCN 43996; Watson et al., GCN 44001; Lai et al., GCN 44002; Izzo et al., GCN 44003; Brivio et al., GCN 44004; Moskvitin et al., GCN 44006; Romanov, GCN 44020; Brosio et al., GCN 44021; Pawar et al., GCN 44022; Belkin et al., GCN 44043; Li et al., GCN 44044; Gupta et al., GCN 44051; Becerra et al., GCN 44059; Volnova et al., GCN 44060; Busmann et al., GCN 44061; Zheng et al., GCN 44065; Brosio et al., GCN 44067; Pankov et al. GCN 44073; Perley et al. GCN 44096; Passaleva et al. GCN 44104)
We report part of our follow-up results in the table below
+------------------+-----------+--------+----------------+-------------+
| Tmid-TGRB (days) | Exp (s) | Filter | Magnitude | Instrument |
+==================+===========+========+================+=============+
| 14.83 | 16 x 300s | r (AB) | 19.96 +/- 0.09 | ODK16 |
| 15.17 | 17 x 180s | r (AB) | 19.82 +/- 0.12 | TEC160FL |
+------------------+-----------+--------+----------------+-------------+
The KNC members will keep monitoring the source in the coming days.
All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained with the sloan filters were calibrated using the PanSTARRS DR1 catalog.
We use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023).
GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/).
GCN Circular 44104
Subject
GRB 260310A: LBT optical observations
Date
2026-03-24T14:51:33Z (6 months ago)
From
N. Passaleva at Sapienza University of Rome <niccolo.passaleva@uniroma1.it>
Via
Web form
Niccolo’ Passaleva, Yu-Han Yang, Narges Shahamt, Eleonora Troja (U Rome) report on behalf of the ERC BHianca team:
Following the detection of GRB 260310A by Fermi/GBM and AstroSat CZTI (Fermi GBM team, GCN 43951,43975; AstroSat CZTI collaboration, GCN 43958), we observed the likely counterpart AT2026fgk (discovered by GOTO; O’Neill et al. 2026, TNS Discovery Report 294132) in r and z filters with the Large Binocular Camera (LBC) mounted on the Large Binocular Telescope (LBT). Observations started at 2026-03-18T09:25:06 (~ 8.2 d after the GBM trigger) with an average airmass of 1.3.
We detect the source with a preliminary magnitude of r~18.7 AB mag, calibrated using nearby stars from the Legacy Survey DR10 (Dey et al. 2019) catalogue, uncorrected for Galactic extinction.
Our measurement is consistent with those at the same observing time reported by Perley et al. (GCN 44096), Volnova et al. (GCN 44060).
We acknowledge excellent support from the LBTO and LBT-INAF staff.
GCN Circular 44096
Subject
GRB260310A: Palomar 60-inch photometry of an extremely bright late-time afterglow with a temporal break at 8 days
Date
2026-03-23T22:07:59Z (6 months ago)
From
Daniel Perley at Liverpool JMU <d.a.perley@ljmu.ac.uk>
Via
Web form
GRB260310A: Palomar 60-inch photometry of an extremely bright late-time afterglow with a temporal break at 8 days
D. A. Perley (LJMU), Y. Wagh (LJMU), R. Jayaraman (Cornell), J. Sollerman (Stockholm), A. Y. Q. Ho (Cornell), K.-R. Hinds (Caltech), and A. Bochenek (LJMU) report:
We used the SED Machine Rainbow Camera optical imager mounted on the Palomar 60-inch telescope to obtain observations of AT2026fgk (Hinds et al., TNS AstroNote 2026-65), the optical counterpart (Konno et al., GCN 43974) of the Fermi/GBM gamma-ray burst GRB260310A (Fermi GBM Team, GCN 43951; Hamburg & Meegan, GCN 43975). Observations began on the night of 2026-03-12 at 06:03 UT and have continued every night since in the SDSS g, r, and i filters; u-band imaging was also taken on certain nights. Photometry was performed following image subtraction of template images taken from Pan-STARRS survey imaging.
For the first seven days after the GRB, the fading is generally consistent with a power-law evolution with a shallow temporal index of approximately -0.6. Beginning at around 7-8 days, the afterglow begins falling more sharply, with a decay index closer to -1.35. (A break in the X-ray light curve was seen at a similar time; Jayaraman et al., GCN 44095 ). Select photometry (AB, not corrected for extinction) is provided below:
| UT date | dt (d) | filter | mag +/- unc |
|---|---|---|---|
| 2026-03-12 06:03:41 | 2.047 | r | 18.14 +/- 0.04 |
| 2026-03-13 06:33:38 | 3.067 | r | 18.19 +/- 0.03 |
| 2026-03-15 06:08:19 | 5.050 | r | 18.59 +/- 0.03 |
| 2026-03-18 06:01:43 | 8.045 | r | 18.83 +/- 0.03 |
| 2026-03-23 05:20:09 | 13.016 | r | 19.58 +/- 0.04 |
The behavior is consistent with other recent reports noting the continuation of the optical decay with no SN-like rebrightening yet (e.g., Volnova et al., GCN 44060; Bussman et al., GCN 44061).
Accounting for Galactic extinction and converting to the equivalent host-frame bandpass, the absolute magnitude of the optical transient as of the most recent observation is M_V ~ -19.7. The fact that the light curve is still decaying steeply suggests that any associated supernova is likely to be significantly dimmer than the benchmark SN1998bw, which peaked at a similar magnitude and phase (e.g., Galama et al. 1998, Nature, 395, 670). The color of the afterglow indicates only modest dust extinction in the host galaxy, so the subluminous nature of the SN is likely intrinsic. A fainter SN comparable to SN2006aj is not yet ruled out, but will be tightly constrained by observations during the next 1-2 weeks.
Additionally, we note that the optical afterglow is both extremely bright and very luminous at late times. To our knowledge only one[1] previous GRB (GRB030329 at z=0.168) has remained brighter than r=20 mag more than 10 observer-frame days after the event, excluding events dominated by the rebrightening of the SN on this timescale (see e.g., Kann et al. 2023, ApJL, 948, 12). The luminosity is also notable: despite its relatively weak prompt emission (E_iso ~ 10^51 erg: Minaev et al., GCN 44053), the afterglow luminosity of GRB260310A at the current time is at the upper end of the afterglow luminosity distribution at this time post-burst, comparable to many high-luminosity (E_iso ~ 10^54 erg) GRBs such as 080319B, 130427A, and 221009A.
We encourage continued monitoring of this exceptional GRB afterglow (and any potential associated SN), which is likely to remain accessible to small- to medium-size optical facilities for many more weeks and possibly months.
[1] GRB221009A would have been observed to have a similar apparent magnitude at this time but was heavily obscured by Galactic dust.
Based on observations obtained with the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Award #2407588 and a partnership including Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, USA; Cornell University, USA; Drexel University, USA; University of North Carolina at Chapel Hill, USA; Institute of Science and Technology, Austria; National Central University, Taiwan, and the German Center for Astrophysics (DZA), Germany. Operations are conducted by Caltech's Optical Observatory (COO), Caltech/IPAC, and the University of Washington at Seattle, USA.
DAP acknowledges the work, legacy, and friendship of D. Alexander Kann, who doubtless would have been tremendously excited by this GRB afterglow.
GCN Circular 44095
Subject
GRB 260310A/AT2026fgk: Detection of temporal steepening in Einstein Probe X-ray data
Date
2026-03-23T21:59:34Z (6 months ago)
From
Rahul Jayaraman at Cornell University <rj438@cornell.edu>
Via
Web form
Rahul Jayaraman (Cornell), Gaurav Waratkar (Caltech), K-Ryan Hinds (Caltech), Yogesh Wagh (LJMU), Daniel Perley (LJMU), Anna Y. Q. Ho (Cornell), S. Y. Fu (HUST), R. D. Liang, M. J. Liu, Z. X. Ling, Hui Sun, Weimin Yuan (NAO, CAS), Genevieve Schroeder (Cornell), Xander J. Hall (CMU), Michael Coughlin (UMN), Aleksandra Bochenek (LJMU), and Jesper Sollerman (Stockholm) report:
The Follow-up X-ray Telescope on board the Einstein Probe has continued to monitor the X-ray afterglow of GRB 260310A (Fermi GBM team, GCN 43951; Arya et al., GCN 43958; Jayaraman et al., GCN 43994), which is associated with the optical transient AT2026fgk (Konno et al., GCN 43974; Hinds et al., AstroNote 2026-65).
Observations with duration ~3 ks were taken at intervals of roughly every 2 days after the initial detection of this source in X-rays (Jayaraman et al., GCN 43994). With four further epochs of data since the initial detection, we find evidence for a steepening in the X-ray light curve at around 8 ± 2 days after the burst, with the decay slope steepening from roughly –0.3 to –1.6. The photon index measured by EP remains at roughly 1.5 throughout these observations.
This steepening appears contemporaneous with an observed steepening at a similar epoch post-burst in r-band photometric data taken by the SED Machine on the Palomar P60 telescope, which will be reported in a forthcoming Circular.
Launched on January 9, 2024, the Einstein Probe is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is supported by the Strategic Priority Program on Space Sciénce of the Chinese Academy of Sciences, in collaboration with the European Space Agency, the Max-Planck-Institute for Extraterrestrial Physics (Germany), and the Centre National d'études Spatiales (France).
GCN Circular 44073
Subject
GRB 260310A: continued optical observation at Mondy
Date
2026-03-21T22:21:34Z (6 months ago)
From
Alexei Pozanenko at IKI, Moscow <grb.alex@gmail.com>
Via
email
N. Pankov (IKI), A. Volnova (IKI), E. Klunko (ISTP), A. S. Moskvitin
(SAO RAS), O. Spiridonova (SAO RAS), A. Pozanenko (IKI) report on
behalf of GRB IKI FuN.
We continued observation the GRB260310A/AT2026fgk detected by
Fermi/GBM (Fermi GBM team, GCN 43951) and AstroSat (Salunke et al.,
GCN 43958) with AZT-33IK telescope of Mondy observatory. We obtained
series in B and R filters on 2026-03-21.
The OT discovered by GOTO (O'Neill et al. 2026, TNS Discovery Report
294132) and observed earlier (Konno et al., GCN 43974; Hinds et al.,
GCN 43977; Lipunov et al., GCN 43978;Martin-Carrillo et al., GCN
43979; Méndez et al., GCN 43980;de Ugarte Postigo et al., GCN 43984;
Hsu et al., GCN 43986; Pursiainen et al., GCN 43990; Becerra et al.,
GCN 43991; Li et al., GCN 43993; Jayaraman et al., GCN 43994; Stein et
al., GCN 43996;Martin-Carrillo et al., GCN 44000; Moreno Méndez et
al., GCN 43980; Becerra et al., GCN 43991; Li et al., GCN 43993; Stein
et al., GCN 43996; Watson et al., GCN 44001; Lai et al., GCN 44002;
Izzo et al., GCN 44003; Brivio et al., GCN 44004; Moskvitin et al.,
GCN 44006; Romanov, GCN 44020; Brosio et al., GCN 44021; Pawar et al.,
GCN 44022; Belkin et al., GCN 44043; Li et al., GCN 44044; Gupta et
al., GCN 44051; Becerra et al., GCN 44059; Volnova et al., GCN 44060;
Busmann et al., GCN 44061; Zheng t al., GCN 44065) is clearly detected
in our images. Preliminary photometry is the following
Date UT_start t_mid - T0, exp, mag, err, filter, UL(3sigma)
days s
2026-03-21 16:35:57 11.48526 30*120 B 20.19 0.04 23.3
2026-03-21 16:04:50 11.46366 15*120 R 19.21 0.04 23.4
The photometry is based on the nearby calibrations stars of PS1
catalogue converted to B, R using the Lupton (2005) transformations.
The magnitudes of the AT2026fgk are not corrected for Galactic
extinctions.
The light curve obtained from our observations and GCNs can be found
in https://heaiki.ru/lvc/GRB260310A/GRB260310A_LC.png
We can assume that the light curve becomes steeper 9 days after long
lasting plateau phase
GCN Circular 44067
Subject
GRB 260310A / AT2026fgk: ABObservatory and NOCTIS new optical observations
Date
2026-03-21T11:45:32Z (6 months ago)
From
A. Brosio at ABObservatory Rosarno <antonino.brosio@gmail.com>
Via
Web form
A. Brosio (ABObservatory Rosarno), M. A. Tripodi (ABObservatory Rosarno), S. Savaglio (Università della Calabria), S. Benatti (INAF - Osservatorio Astronomico di Palermo), M. Basilicata, M. Rainer (INAF - Osservatorio Astronomico di Brera), S. Tosi (Università di Genova), A. Di Dato (INAF - Osservatorio Astronomico di Capodimonte), D. Liguori (Osservatorio "G. Galilei", Liceo Scientifico Cariati), A. Nastasi (GAL Hassin, Isnello), E. Pace, R. Stanga, L. Betti (Università di Firenze - Osservatorio Polifunzionale del Chianti), D. Ricci (INAF - Osservatorio Astronomico di Padova) and the NOCTIS Collaboration report:
We observed the optical counterpart of GRB 260310A / AT 2026FGK, previously reported by several groups as the optical afterglow:
(O'Neill et al. 2026, TNS Discovery Report 294132; Konno et al., GCN 43974; Hinds et al., GCN 43977; Lipunov et al., GCN 43978; Martin-Carrillo et al., GCN 43979; Méndez et al., GCN 43980; de Ugarte Postigo et al., GCN 43984; Hsu et al., GCN 43986; Pursiainen et al., GCN 43990; Becerra et al., GCN 43991; Li et al., GCN 43993; Jayaraman et al., GCN 43994; Stein et al., GCN 43996; Martin-Carrillo et al., GCN 44000; Watson et al., GCN 44001; Lai et al., GCN 44002; Izzo et al., GCN 44003; Brivio et al., GCN 44004; Moskvitin et al., GCN 44006; Romanov, GCN 44020; Pawar et al., GCN 44022; Belkin et al., GCN 44043; Li et al., GCN 44044; Gupta et al., GCN 44051; Salunke et al., GCN 43958; Becerra et al., GCN 44059; Volnova et al., GCN 44060; Busmann et al., GCN 44061).
At JD 2461120.34019 (2026-03-20.84019 UT), we measured the source at:
AT 2026FGK 18.92 +/- 0.10 Sloan r
Photometric calibration was performed using the APASS catalog, adopting the comparison stars at coordinates
RA(J2000) = 219.274765, Dec(J2000) = +71.914946;
RA(J2000) = 219.311345, Dec(J2000) = +71.880330.
GCN Circular 44065
Subject
GRB 260310A / AT 2026fgk: KAIT optical observations
Date
2026-03-21T06:49:53Z (6 months ago)
From
Weikang Zheng at UC Berkeley <weikang@berkeley.edu>
Via
email
WeiKang Zheng (UCB) and Alexei V. Filippenko (UCB) report on
behalf of the KAIT GRB team:
The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at
Lick Observatory, observed the field of GRB 251022A (Fermi GBM
Team, GCN 43951; Salunke et al., GCN 43958) on March 13, 14, 18
19, 20 UT. A set of B, V, R, I and clear (roughly R) filter images
were obtained. We clearly detected the optical afterglow (O'Neill
et al. 2026, TNS Discovery Report 294132; Konno et al., GCN 43974;
Hinds et al., GCN 43977; Lipunov et al., GCN 43978;Martin-Carrillo
et al., GCN 43979; Méndez et al., GCN 43980;de Ugarte Postigo
et al., GCN 43984; Hsu et al., GCN 43986; Pursiainen et al., GCN
43990; Becerra et al., GCN 43991; Li et al., GCN 43993; Jayaraman
et al., GCN 43994; Stein et al., GCN 43996;Martin-Carrillo et al.,
GCN 44000; Moreno Méndez et al., GCN 43980; Becerra et al., GCN
43991; Li et al., GCN 43993; Stein et al., GCN 43996; Watson et al.,
GCN 44001; Lai et al., GCN 44002; Izzo et al., GCN 44003; Brivio
et al., GCN 44004; Moskvitin et al., GCN 44006; Romanov, GCN 44020;
Brosio et al., GCN 44021; Pawar et al., GCN 44022; Belkin et al.,
GCN 44043; Li et al., GCN 44044; Gupta et al., GCN 44051; Salunke
et al., GCN 43958; Becerra et al., GCN 44059; Volnova et al., GCN
44060; Busmann et al., GCN 44061) in our coadd images and photometry
in R band gives the following result:
t-t0(d) Mag err
------------------
3.21 17.98 0.03
4.25 18.31 0.02
8.24 18.34 0.02
9.22 18.30 0.02
10.20 18.54 0.03
GCN Circular 44063
Subject
GRB 260310A / AT2026fgk: NuSTAR follow-up detection
Date
2026-03-21T04:50:06Z (6 months ago)
From
Gaurav Waratkar at Caltech <gauravwaratkar@iitb.ac.in>
Via
Web form
Elias Kammoun (Caltech), Gaurav Waratkar (Caltech), Rahul Jayaraman (Cornell), K-Ryan Hinds (Caltech), Anna Y. Q. Ho (Cornell) report:
In response to our NuSTAR DDT request (PI Waratkar), NuSTAR observed the position of GRB 260310A / AT2026fgk (Fermi GBM team, GCN 43951; Arya et al., GCN 43958; Konno et al., GCN 43974; Martin-Carrillo et al., GCN 43979, 44000; Méndez et al., GCN 43980; Becerra et al., GCN 43991; Jayaraman et al., GCN 43994; Stein et al., GCN 43996; Watson et al., GCN 44001; Lai et al., GCN 44002; Izzo et al., GCN 44003; Brivio et al., GCN 44004; Rhodes et al., GCN 44005; Moskvitin et al., GCN 44006; Romanov et al., GCN 44020; Brosio et al., GCN 44021; Pawar et al., GCN 44022; Belkin et al., GCN 44043; Li et al., GCN 44044; Giarratana et al., GCN 44045; Gupta et al., GCN 44051; Ho et al., GCN 44057; Passaleva et al., GCN 44058; Becerra et al., GCN 44059; Volnova et al., GCN 44060), beginning at 2026-03-20T17:20:02 UTC with an exposure time of 7523 s.
We performed a preliminary analysis of the NuSTAR spectrum in the 3-79 keV energy range from both the FPMA and FPMB modules by fitting an absorbed power law. We find a photon index of 1.76 ± 0.22 with an unabsorbed flux of (1.16 ± 0.15) x 10^-12 ergs/cm2/s (2-10 keV). All the uncertainties are 90% CL. At the reported redshift of 0.153 (Ugarte Postigo et al., GCN 43984; Hinds et al., GCN 43977), this would correspond to a luminosity (2-10 keV) of (7.2 ± 0.7) x 10^43 erg/s.
We thank the entire NuSTAR Science & Mission operations teams for the rapid approval and execution of this DDT observation. NuSTAR is a NASA Small Explorer mission led by Caltech and managed by JPL for NASA's Science Mission Directorate in Washington.
GCN Circular 44061
Subject
GRB 260310A / AT 2026fgk: FTW optical and NIR observations show no rebrightening
Date
2026-03-21T01:42:34Z (6 months ago)
Edited On
2026-03-21T17:42:59Z (6 months ago)
From
Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>
Via
Web form
Malte Busmann, Ziyuan Zhu, Mitra Maleki (LMU), Julius Gassert (LMU/CMU), Daniel Gruen (LMU), Xander J. Hall, Brendan O'Connor, Antonella Palmese (CMU) report:
We observed the optical/NIR counterpart of GRB 260310A (Fermi GBM team, GCN 43951; Salunke et al., GCN 43958), AT 2026fgk (e.g. O'Neill et al. 2026, TNS Discovery Report 294132; Konno et al., GCN 43974; Hinds et al., GCN 43977), with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the g, r, i, z, J, and Ks band starting at 2026-03-19T23:22:38 (9.77 days after the GRB trigger) and 2026-03-20T22:36:22 (10.74 after the GRB trigger). Unlike suggested by Becerra et al. (CGN 44059) we do not see signs of rebrightening in our observations in any filter (see also Volnova et al., GCN 44060).
We thank Michael Schmidt and Silona Wilke for obtaining these observations.
GCN Circular 44060
Subject
GRB 260310A: no evidence of re-brightening up to 9.6 days in optical observation at Mondy
Date
2026-03-20T22:31:46Z (6 months ago)
From
Alexei Pozanenko at IKI, Moscow <grb.alex@gmail.com>
Via
email
A. Volnova (IKI), N. Pankov (IKI), E. Klunko (ISTP), A. S. Moskvitin
(SAO RAS), O. Spiridonova (SAO RAS), A. Pozanenko (IKI) report on
behalf of GRB IKI FuN.
We observed the field of GRB260310A/AT2026fgk detected by Fermi/GBM
(Fermi GBM team, GCN 43951) and AstroSat(Salunke et al., GCN 43958)
with AZT-33IK telescope of Mondy observatory. We obtained series in B
and R filters starting on 2026-03-13.
The OT discovered by GOTO (O'Neill et al. 2026, TNS Discovery Report
294132) and observed earlier (Konno et al., GCN 43974; Hinds et al.,
GCN 43977; Lipunov et al., GCN 43978;Martin-Carrillo et al., GCN
43979; Méndez et al., GCN 43980;de Ugarte Postigo et al., GCN 43984;
Hsu et al., GCN 43986; Pursiainen et al., GCN 43990; Becerra et al.,
GCN 43991; Li et al., GCN 43993; Jayaraman et al., GCN 43994; Stein et
al., GCN 43996;Martin-Carrillo et al., GCN 44000; Moreno Méndez et
al., GCN 43980; Becerra et al., GCN 43991; Li et al., GCN 43993; Stein
et al., GCN 43996; Watson et al., GCN 44001; Lai et al., GCN 44002;
Izzo et al., GCN 44003; Brivio et al., GCN 44004; Moskvitin et al.,
GCN 44006; Romanov, GCN 44020; Brosio et al., GCN 44021; Pawar et al.,
GCN 44022; Belkin et al., GCN 44043; Li et al., GCN 44044; Gupta et
al., GCN 44051) is clearly detected in our images. Preliminary PSF
photometry on some epochs is the following
Date UT_start t_mid - T0, filter exp, mag err UL(3sigma)
days s
2026-03-15 15:09:04 5.43535 15*120 B 19.18 0.05 23.0
2026-03-16 14:37:32 6.41345 15*120 B 19.05 0.05 22.6
2026-03-17 18:59:03 7.60549 30*120 B 19.18 0.05 22.7
2026-03-18 18:40:29 8.58565 19*120 B 19.47 0.06 21.7
2026-03-19 18:09:48 9.57128 30*120 B 19.61 0.06 23.2
2026-03-15 14:48:55 5.52493 11*120 R 18.27 0.05 23.0
2026-03-16 14:07:40 6.39202 13*120 R 18.22 0.05 22.8
2026-03-17 18:22:13 7.56948 15*120 R 18.29 0.06 22.8
2026-03-18 18:06:29 8.55786 15*120 R 18.60 0.06 22.4
2026-03-19 19:11:16 9.60355 15*120 R 18.71 0.07 22.7
The photometry is based on the nearby calibrations stars of PS1
catalogue converted to B, R using the Lupton (2005) transformations.
The magnitudes are not corrected for Galactic extinctions.
We do not observe the rebrightening reported in COLIBRÍ observations
(Becerra et al., GCN 44059) up to the latest epoch (~9.6 days) in B,R
- filters.
Based on classification of the progenitor (Atteia et al., GCN 43981;
Minaev et al., GCN 44053) we urge follow up observations to search for
a supernova associated with GRB 260310A.
GCN Circular 44059
Subject
GRB 260310A / AT 2026fgk: COLIBRÍ evidence of a late rebrightening
Date
2026-03-20T20:08:08Z (6 months ago)
From
Rosa L. Becerra at Instituto de Astronomía, UNAM <rbecerra@astro.unam.mx>
Via
Web form
Rosa L. Becerra (UNAM), Camila Angulo (UNAM), Sarah Antier (IJCLAB), Noémie Globus (UNAM), Enrique Moreno Méndez (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Antonio de Ugarte Postigo (LAM), and Benjamin Schneider (LAM) report:
We reimaged the field of AT 2026fgk, first reported by Hinds et al. (AstroNote 2026-65) and followed-up by a large number of facilities across multiple wavelengths (GCNs Circ. 43951, 43954, 43958, 43974, 43975, 43977, 43978, 43979, 43980, 43981, 43984, 43986, 43990, 43991, 43993, 43994, 43996, 44000, 44001, 44002, 44003, 44004, 44005, 44006, 44020, 44021, 44022, 44043, 44044, 44045, 44051, 44053, 44057, 44058), as a possible counterpart to the Fermi GBM GRB 260310A (Fermi GBM Team, GCN Circ. 43951; Humburg & Meegan, GCN Circ. 43975), and also detected by AstroSat CZTI (Salunke et al., GCN Circ. 43958), using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed several epochs from 2026-03-20 03:35 to 11:21 UTC (from 9.94 to 10.27 days after the GRB trigger) in the g, r, and z filters.
The data were reduced, coadded, calibrated, and analysed with the ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.
In the stacked images, we continue to detect AT 2026fgk. Compared with the photometry acquired on 2026-03-19, we observe a rebrightening in all three filters throughout the night, with a temporal index of alpha > 1.5. We encourage extended follow-up observations.
We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM.
COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.
GCN Circular 44058
Subject
GRB 260310A/AT2026fgk: LBT near-infrared observations
Date
2026-03-20T17:40:46Z (6 months ago)
From
N. Passaleva at Sapienza University of Rome <niccolo.passaleva@uniroma1.it>
Via
Web form
Niccolo’ Passaleva, Yu-Han Yang, Narges Shahamt, Eleonora Troja (U Rome) report on behalf of the ERC BHianca team:
Following the detection of GRB 260310A by Fermi/GBM and AstroSat CZTI (Fermi GBM team, GCN 43951,43975; AstroSat CZTI collaboration, GCN 43958), we observed the likely counterpart AT2026fgk (discovered by GOTO; O’Neill et al. 2026, TNS Discovery Report 294132) in J and K filters with the LUCI near-infrared imager mounted on the Large Binocular Telescope (LBT). Observations started at 2026-03-16T08:07:32 (~ 6.2 d after the GBM trigger) with an average airmass of 1.3.
We detect the source with a preliminary magnitude of J~18.03 AB mag, calibrated using nearby stars from the 2MASS catalog (Skrutskie et al. 2006), uncorrected for Galactic extinction.
Compared with previous near-infrared observations (Li et al., GCN 43993; Stein et al., GCN 43996; Brivio et al., GCN 44004), our measurement is consistent with a fading slope of 0.7.
We acknowledge excellent support from the LBTO and LBT-INAF staff.
GCN Circular 44057
Subject
NOEMA Detection of GRB 260310A / AT 2026fgk
Date
2026-03-20T14:52:58Z (6 months ago)
Edited On
2026-03-20T15:29:02Z (6 months ago)
From
Anna Y Q Ho at Cornell University <ayh24@cornell.edu>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Anna Y Q Ho at Cornell University <ayh24@cornell.edu>
Via
Web form
Anna Y. Q. Ho (Cornell), Michael Bremer (IRAM), Genevieve Schroeder (Cornell) report on behalf of a larger collaboration:
On UT 2026 March 20 the NOrthern Extended Millimeter Array (NOEMA) observed the position of GRB 260310A / AT2026fgk (GCN 43951, 43958, 43974), a nearby (z=0.153; GCN 43977) gamma-ray burst with bright radio emission at cm wavelengths (GCN 44005, 44045). Observations were obtained in the new dual-band mode (100+230 GHz) as well as in Band 2 (150 GHz). With self-calibration, we measure a preliminary flux density of 17 mJy at 100 GHz, corresponding to a luminosity of 10^42 erg/s, typical for GRB afterglows at this epoch.
Further observations are planned. We thank the NOEMA staff for rapidly approving and scheduling these observations.
GCN Circular 44053
Subject
GRB 260310A: classification as long GRB based on Eiso, Ep, and T_90
Date
2026-03-20T01:38:39Z (6 months ago)
From
Alexei Pozanenko at IKI, Moscow <grb.alex@gmail.com>
Via
email
P. Minaev (IKI) , A. Pozanenko (IKI) on behalf of GRB follow-up team
We analyzed GRB 260310A (Hamburg et al., GCN 43975; Salunke et al.,
GCN 43958; Atteia et al., GCN 43981) using publicly available data of
GBM/Fermi. The burst consists of several emission episodes with a
total duration of T_90 = 86 s. Time integrated energy spectrum,
constructed in the time interval (-4, 70) s since GBM trigger, is best
described by Band model with E_p = 175 keV and fluence F = 1.74e-5 erg
cm^-2 in 1 keV - 10 MeV range. Using z = 0.153 (Hinds et al., GCN
43977; de Ugarte Postigo et al., GCN 43984) we obtain E_iso = 9.8e50
erg. GRB 260310A is placed above the correlation fit for type II
(long) bursts on Ep,i – Eiso diagram, as mentioned in (Atteia et al.,
GCN 43981), but it is inside 2 sigma correlation regions for both type
I (short) and type II (long) bursts, therefore we cannot consider it
as a clear outlier. Indeed, at the EH – T_90,i diagram (see [1-2] for
details) GRB 260310A is well classified as type II (long) burst. The
Ep,i – Eiso and EH - T90,i diagrams can be found at
https://heaiki.ru/lvc/GRB260310A/
[1] - Minaev et al., MNRAS, 492, 1919, 2020
[2] - Minaev et al., Astronomy Letters, 46, 9, 573, 2020
GCN Circular 44051
Subject
GRB260310A/AT2026fgk: Further optical observation from 1.3m DFOT
Date
2026-03-19T14:57:05Z (6 months ago)
From
ANSHIKA GUPTA at ARIES <anshika05180@gmail.com>
Via
Web form
Anshika Gupta, Kumar Pranshu, Dhruv Jain, Debalina Kar, Pankaj Pawar, and Kuntal Misra (ARIES) report:
We observed the field of GRB260310A/AT2026fgk detected by Fermi/GBM (Fermi GBM team, GCN 43951) and AstroSat(Salunke et al., GCN 43958) with the 1.3m Devasthal Fast Optical Telescope (DFOT), located at the Devasthal Observatory of the Aryabhatta Research Institute of Observational Sciences (ARIES), India. The observations began on 2026-03-17 at 20:33:36.00 UT, i.e., ~ 7.65 days after the Fermi/GBM trigger. We have taken multiple frames with an exposure time of 300s in the I filter. We stacked the images after the alignment. We detect an optical afterglow in our stacked image. We obtain the following preliminary magnitude in the stacked image:
Date Start_UT T_start-T0 (days) Filter Exp time (s) Magnitude
================================================================
2026-03-17 20:33:36.00 ~7.65 I 300s*3 18.01 +/-0.02
The optical detection of the burst is consistent with Lipunov et al., GCN 43954; Konno et al., GCN 43974; Hinds et al., GCN 43977; Lipunov et al., GCN 43978; Martin-Carrillo et al., GCN 43979, 44000; Moreno Méndez et al., GCN 43980; Becerra et al., GCN 43991; Li et al., GCN 43993; Stein et al., GCN 43996; Watson et al., GCN 44001; Lai et al., GCN 44002; Izzo et al., GCN 44003; Brivio et al., GCN 44004; Moskvitin et al., GCN 44006; Romanov, GCN 44020; Brosio et al., GCN 44021; Pawar et al., GCN 44022; Belkin et al., GCN 44043; Li et al., GCN 44044.
The magnitude is not corrected for the Galactic extinction in the direction of the burst.
Photometric calibration is performed using the standard stars from the USNO-B1.0 catalogue.
GCN Circular 44045
Subject
GRB 260310A / AT 2026fgk: radio detection with the VLA
Date
2026-03-18T09:14:01Z (6 months ago)
From
Stefano Giarratana at INAF-OAB <s.giarratana@ira.inaf.it>
Via
email
S. Giarratana (INAF-OAB), M. Giroletti (INAF-IRA),
G. Ghirlanda (INAF-OAB), N. Di Lalla (Stanford Univ.),
N. Omodei (Stanford Univ.), O. S. Salafia (INAF-OAB),
L. Nava (INAF-OAB)
At 10:22:44 UT on 2026 March 14 (T_mid = 4.26 days
post-burst) the Karl G. Jansky VLA observed the field of
GRB 260310A / AT 2026fgk (Fermi GBM team, GCN 43951,
43975; AstroSat CZTI collaboration, GCN 43958) in three
bands, with central frequencies of 6, 10 and 15 GHz.
The standard 3C286 was used as bandpass and flux density
calibrator, while J1435+7605 served as phase calibrator.
From a preliminary analysis, an unresolved radio source
(Rhodes et al., GCN 44005) is clearly detected at all
frequencies at a position:
RA: 14:37:16.1424 +- 0.001
Dec: +71:50:30.328 +- 0.01
consistent with the X-ray (Jayaraman et al., GCN 43994) and
optical (Hinds et al., AstroNote 2026-65, GCN 43977;
Konno et al., GCN 43977; Lipunov et al., GCN 43978;
Martin-Carrillo et al., GCN 43979, 44000; Moreno Méndez
et al, GCN 43980; Becerra et al., GCN 43991; Watson et al.,
GCN 44001; Lai et al., GCN 44002; Izzo et al., GCN 44003;
Moskvitin et al., GCN 44006; Romanov, GCN 44020; Brosio et
al., GCN 44021; Pawar et al., GCN 44022) and infrared
(Li et al., GCN 43993; Stein et al., GCN 43996; Brivio et
al., GCN 44004) position of the transient.
The preliminary analysis yields the following results:
================================================================
T_mid Freq Peak r.m.s. Beam PA
[days] [GHz] [uJy/b] [uJy/b] [arcsec^2] [deg]
================================================================
4.26 6 3894 8 0.39x0.29 -31
4.26 10 6368 13 0.24x0.19 3.9
4.26 15 9104 29 0.16x0.11 -11
================================================================
We would like to thank the staff of the VLA for approving, executing,
and processing the observations.
The National Radio Astronomy Observatory is a facility of the National
Science Foundation operated under cooperative agreement by Associated
Universities, Inc.
These observations were carried out as part of project SF181027,
approved in the framework of the Fermi - NRAO joint program agreement.
GCN Circular 44044
Subject
GRB 260310A / AT2026fgk: SVOM/VT optical observations
Date
2026-03-18T07:47:29Z (6 months ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H. L. Li, L. P. Xin, Y. N. Ma, C. Wu, Z. H. Yao, Y. L. Qiu, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. R. Xu, J. S. Deng, J. Y. Wei (NAOC), J. T. Palmerio (CEA/Irfu)，J. X. Cao (GXU) report on behalf of the SVOM/VT team.
SVOM/VT performed several ToO observations of the field of GRB 260310A detected by Fermi (Fermi GBM team, GCN 43951) and AstroSat(Salunke et al., GCN 43958) from 2026-03-14 to 2026-03-17 in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.
The reported optical counterpart AT2026fgk discovered by GOTO(O'Neill et al. 2026, TNS Discovery Report 294132) and observed by other observatories (Lipunov et al., GCN 43954; Konno et al., GCN 43974, Hinds et al., GCN 43977; Lipunov et al., GCN 43978; Martin-Carrillo et al., GCN 43979, Moren Méndez et al., GCN 43980; de Ugarte Postigo et al., GCN 43984, Pursiainen et al., Becerra et al., GCN 43991; Martin-Carrillo et al., GCN 44000; Watson et al., GCN 44001; Lai et al., GCN 44002; L. Izzo, GCN 44003; Brivio et al., GCN 44004; Rhodes et al., GCN 44005; Moskvitin et al., GCN 44006; Brosio et al., GCN 44021; Pawar et al., GCN 44022; Belkin and Shrestha, GCN 44043) is clearly detected in single frames in both channels. The preliminary measurements in AB magnitudes are given below:
Mid_time Band Exposure Time Magnitude (AB)
5.26 day VT_R 70 sec 18.44 +/- 0.05 mag
5.26 day VT_B 70 sec 18.94 +/- 0.05 mag
7.35 day VT_R 70 sec 18.65 +/- 0.05 mag
7.35 day VT_B 70 sec 19.15 +/- 0.05 mag
Our photometry was not corrected for Galactic extinction.
The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.
GCN Circular 44043
Subject
GRB 260310A / AT2026fgk: LCO optical observations
Date
2026-03-18T06:59:09Z (6 months ago)
From
Sergey Belkin at Monash University <sergey.belkin@monash.edu>
Via
Web form
S. Belkin and M. Shrestha (Monash U) report:
We observed the optical counterpart AT2026fgk of GRB 260310A, detected by Fermi/GBM (Fermi GBM Team, GCN 43951) and AstroSat/CZTI (Salunke et al., GCN 43958), with the Las Cumbres Observatory 1m telescope at McDonald Observatory equipped with the Sinistro instrument.
Observations began on 2026-03-14 at 06:12:09 UT, approximately 4.05 days after the trigger. We obtained 4 x 60 s exposures in the r band and 6 x 60 s exposures in the u band. In the stacked images we clearly detect the reported optical counterpart AT2026fgk (e.g. Lipunov et al., GCN 43954; Konno et al., GCN 43974; Hinds et al., GCN 43977; Lipunov et al., GCN 43978; Martin-Carrillo et al., GCN 43979; Moreno Mendez et al., GCN 43980; de Ugarte Postigo et al., GCN 43984; Pursiainen et al., GCN 43990; Becerra et al., GCN 43991; Martin-Carrillo et al., GCN 44000; Watson et al., GCN 44001; Lai et al., GCN 44002; Izzo, GCN 44003; Moskvitin et al., GCN 44006; Romanov, GCN 44020; Brosio et al., GCN 44021; Pawar et al., GCN 44022), discovered by GOTO (O’Neill et al., TNS Discovery Report 294132).
The preliminary photometry from the stacked images is:
Date (UT) T-T0 (days) Filter Exp. time (s) Magnitude 3-sigma UL
========================================================================================
2026-03-14 06:14:13 4.0535 r 60 x 4 18.58 +/- 0.02 21.79
2026-03-14 06:21:49 4.0588 u 60 x 6 19.47 +/- 0.04 21.12
The r-band photometry is calibrated against Pan-STARRS field stars. The u-band photometry is calibrated against Gaia DR3 synthetic SDSS u-band stars. The quoted values are not corrected for Galactic extinction. Further follow-up is encouraged.
GCN Circular 44022
Subject
GRB 260310A / AT2026fgk: 1.3m DFOT optical observations
Date
2026-03-15T17:21:19Z (6 months ago)
From
pawarpankaj1492@gmail.com
Via
Web form
Pankaj Pawar, Debalina Kar, Anshika Gupta, Dhruv Jain and Kuntal Misra (ARIES) report:
We observed the likely optical counterpart of GRB 260310A, detected by Fermi/GBM(Fermi/GBM team, GCN 43951) and AstroSat(Salunke et al., GCN 43958), with the 1.3m Devasthal Fast Optical Telescope (DFOT), located at the Devasthal Observatory of the Aryabhatta Research Institute of Observational Sciences (ARIES), India.
The observations began on 2026-03-14 at 18:29:38 UT, approximately 109.54 hours (~4.56 days) after the trigger. The reported optical couterpart AT2026fgk (Lipunov et al., GCN 43954; Konno et al., GCN 43974, Hinds et al., GCN 43977; Lipunov et al., GCN 43978; Martin-Carrillo et al., GCN 43979, Moren Méndez et al., GCN 43980; de Ugarte Postigo et al., GCN 43984, Pursiainen et al., Becerra et al., GCN 43991; Martin-Carrillo et al., GCN 44000; Watson et al., GCN 44001; Lai et al., GCN 44002; L. Izzo, GCN 44003; Brivio et al., GCN 44004; Rhodes et al., GCN 44005; Moskvitin et al., GCN 44006; Brosio et al., GCN 44021) is clearly detected in a single 100s R-band frame.
We derive the following magnitude from the stacked image:
Date Start_UT T_start-T0(days) Filter Exp time (s) Magnitude
===============================================================
2026-03-14 18:29:38 ~4.56 R 100s*1 18.54 ± 0.12
The reported magnitude is calibrated using Pan-STARRS field stars and
is not corrected for Galactic extinction.
GCN Circular 44021
Subject
GRB 260310A / AT2026fgk: ABObservatory and NOCTIS optical observations
Date
2026-03-15T16:12:58Z (6 months ago)
From
A. Brosio at ABObservatory Rosarno <antonino.brosio@gmail.com>
Via
Web form
A. Brosio (ABObservatory Rosarno), M. A. Tripodi (ABObservatory Rosarno), S. Savaglio (Università della Calabria), S. Benatti, M. G. Guarcello (INAF - Osservatorio Astronomico di Palermo), M. Basilicata, L. Cabona, M. Rainer, F. M. Zerbi (INAF - Osservatorio Astronomico di Brera), G. Bracco, P. Cianfarra, L. Sangaletti, S. Tosi, S. Zappatore (Università di Genova), A. Di Dato (INAF - Osservatorio Astronomico di Capodimonte), D. Liguori (Osservatorio "G. Galilei", Liceo Scientifico Cariati), S. Masiero, A. Nastasi (GAL Hassin, Isnello), R. Nesci (Associazione Astronomica Antares APS di Foligno), L. Betti, E. Pace, R. Stanga (Università di Firenze), D. Ricci (INAF - Osservatorio Astronomico di Padova) for the NOCTIS team report
We observed the transient AT 2026fgk, proposed as the likely optical counterpart of GRB 260310A (Konno et al., GCN 43974; Hamburg & Meegan,GCN 43975).
Observations were carried out on 2026-03-14.80913 UT with the 0.30-m f/4 telescope at ABObservatory in clear band
The obtained photometry is:
2026-03-14.80913 UT JD 2461114.30913 m_CV = 18.50 +/- 0.04 mag
The measurement was calibrated against two APASS catalog stars,selected using Aladin. The reported magnitude is in the CV system.
This measurement confirms that the optical counterpart is still clearly detected about 4.6 days after the GRB trigger.
GCN Circular 44020
Subject
GRB 260310A / AT 2026fgk: iTelescope optical observations
Date
2026-03-15T15:06:06Z (6 months ago)
From
Filipp Dmitrievich Romanov at Amateur astronomer <filipp.romanov.27.04.1997@gmail.com>
Via
email
I observed the field of GRB 260310A = AT 2026fgk (Fermi GBM Team, GCN
Circ. #43951) remotely using iTelescope.Net T21 (0.43-m f/6.8
reflector + CCD + f/4.5 focal reducer) in Utah Desert Remote
Observatory at Great Basin Desert, Beryl Junction, Utah, USA.
Six images with exposures of 300 sec. and Rc filter were obtained on
2026-03-15 from 08:56:17 UT (5.166 d. after the trigger) to 09:32:41
UT (5.191 d. after the trigger). I detected the optical afterglow and
measured its magnitude (compared to Rc magnitudes of nearby stars from
the ATLAS star catalog: Tonry et al., 2018) in the stacked image =
18.0 +/- 0.1 Rc.
Magnitude is not corrected for Galactic extinction. The magnitude may
be affected by contamination from the host galaxy.
Stacked image: https://www.flickr.com/photos/filipp-romanov/55149149955/
F. D. Romanov (AAVSO).
GCN Circular 44006
Subject
GRB 260310A / AT2026fgk: SAO RAS optical observations
Date
2026-03-14T15:46:52Z (6 months ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
Web form
A. Moskvitin (SAO RAS), O. Spiridonova (SAO RAS),
A. Pozanenko (IKI), N. Pankov (IKI), A. Volnova (IKI),
report on behalf of GRB follow-up team and IKI-GRB-FuN.
We observed the field of the AT2026fgk, potential counterpart
of the GRB 260310A (Fermi GBM team, GCN 43951; Hamburg and Meegan,
GCN 43975; Salunke et al., GCN 43958) with the Zeiss-1000
1m telescope of the SAO RAS equipped with the CCD photometer.
We obtained series of 300 sec images in B, V, Rc, Ic filters
on March 12/13 and 13/14. The observations carried out
under a good weather conditions and typical seeing of 1".5--2".5.
The candidate discovered by GOTO (O'Neill et al. 2026, TNS Discovery
Report 294132) and observed by numerous observatories (Konno et al.,
GCN 43974; Hinds et al., GCN 43977; Lipunov et al., GCN 43978;
Martin-Carrillo et al., GCN 43979; Moren Méndez et al., GCN 43980;
de Ugarte Postigo et al., GCN 43984; Hsu et al., GCN 43986;
Pursiainen et al., GCN 43990; Becerra et al., GCN 43991; Li et al.,
GCN 43993; Jayaraman et al., GCN 43994; Stein et al., GCN 43996;
Martin-Carrillo et al., GCN 44000; Watson et al., GCN 44001;
Lai et al., GCN 44002; Izzo, GCN 44003; Brivio et al., GCN 44004;
Rhodes et al., GCN 44005) is clearly detected in our images
with the following brightness.
Date UT_start t_mid - T0, filter exp, mag +/- err
days s
2026.03.12T23:27:00 2.80565 B 5*300 18.77 +/- 0.01
2026.03.12T23:09:50 2.79577 Rc 6*300 18.00 +/- 0.01
2026.03.13T23:56:42 3.82450 B 5*300 18.92 +/- 0.01
2026.03.13T23:45:30 3.81664 Rc 4*300 18.14 +/- 0.01
This preliminary photometry is based on the nearby stars from
PanSTARRS catalogue. Magnitudes of standards converted to B, Rc
with the Lupton 2005 equations. The magnitudes of the AT2026fgk
are not corrected for Galactic and host extinction
and may be contaminated by the host galaxy.
We can not trace any colour evolution between the two epochs
(2.8 and 3.8 days after GRB trigger).
GCN Circular 44005
Subject
GRB 260310A/AT2026fgk: radio detection at 15GHz
Date
2026-03-14T15:23:56Z (6 months ago)
Edited On
2026-03-14T19:29:19Z (6 months ago)
From
Lauren Rhodes at McGill <lauren.rhodes@mcgill.ca>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Lauren Rhodes at McGill <lauren.rhodes@mcgill.ca>
Via
Web form
Lauren Rhodes (McGill), Andrew Hughes, Rob Fender (Oxford), Joe Bright (Oxford/Breakthrough Listen), Dave Green, Dave Titterington (Cambridge) report:
We observed the position of GRB 260310A/AT 2026fgk (Fermi GBM Team, GCN 43951; Konno et al., GCN 43974; Hinds et al., AstroNote 2026-65; O’Neill et al. 2026, TNS Discovery Report 294132) with the Arcminute Microkelvin Imager Large-Array (AMI-LA) at 15.2 GHz beginning at 2026 March 13 23:20:06 for a total of 8 hours. The flux standard 3c286 was used to calibrate the bandpass response and flux scale of the AMI-LA and J1435+7605 was used as an interleaved complex gain calibrator.
We detect an unresolved source with coordinates consistent with those of GRB 260310A/AT 2026fgk. We measure a peak flux density of 9.8 +/- 0.5 mJy/beam. Given the high flux density of the source, we cross checked with the VLASS survey and find no preexisting radio emission at the afterglow coordinates down to a 3sigma upper limit of around 0.5mJy/beam.
We thank the staff at the Mullard Radio Astronomy Observatory for carrying out these observations and operating the AMI-LA.
GCN Circular 44004
Subject
GRB 260310A / AT2026fgk: TNG NIR detection
Date
2026-03-14T14:48:10Z (6 months ago)
From
Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>
Via
Web form
R. Brivio (INAF-OAB), D. Malesani (DAWN/NBI and Radboud), L. Izzo (INAF-OACn and DARK/NBI), P. D’Avanzo (INAF-OAB), V. D’Elia (ASI-SSDC), A. Melandri (INAF-OAR), A. Rossi (INAF-OAS), M. de Pasquale (Univ. Messina), Aldo Fiorenzano (INAF-TNG), Carmen Padilla (INAF-TNG) report on behalf of the CIBO collaboration:
We observed the likely counterpart of GRB 260310A, detected by Fermi/GBM (Fermi GBM team, GCN 43951; Hamburg & Meagan, GCNC 43975) and AstroSat CZTI (Salunke et al., GCN 43958) with the Italian 3.6m TNG telescope equipped with the near-infrared camera NICS. A series of images was obtained with the J and H filters starting on 2026 March 14 at 05:11:46 UT (i.e. 96.2 hours after the burst), and lasted 40 minutes for each filter.
The optical/NIR counterpart, also reported as AT2026fgk (Konno et al., GCN 43974; Hinds et al., GCN 43977; Lipunov et al., GCN 43978; Martin-Carrillo et al., GCN 43979; Moreno Méndez et al., GCN 43980; de Ugarte Postigo et al., GCN 43984; Hsu et al., GCN 43986; Pursiainen et al., GCN 43990; Becerra et al., GCN 43991; Li et al., GCN 43993; Stein et al, GCN 43996; Martin-Carrillo et al., GCN 44000; Watson et al., GCN 44001; Lai et al., GCN 44002; L. Izzo, GCN 44003) is detected in the co-added images with the following magnitudes:
J = 17.0 +/- 0.1 (Vega, calibrated against the 2MASS catalogue),
at a mid-time of 96.5 hr after the trigger;
H = 15.9 +/- 0.1 (Vega, calibrated against the 2MASS catalogue),
at a mid-time of 97.1 hr after the trigger.
GCN Circular 44003
Subject
GRB 260310A / AT2026fgk: OASDG optical observations
Date
2026-03-14T09:12:46Z (6 months ago)
From
luca.izzo@inaf.it
Via
Web form
L. Izzo (INAF-OACn and DARK/NBI) reports:
We observed the likely counterpart of GRB 261013A (Fermi GBM team, GCN #43951; Hamburg et al., GCN #43975) using the 0.5-m T1 telescope of the Osservatorio Astronomico S. Di Giacomo, located in Agerola, Italy. Our observations started on 2026 March 14 at 00:04 UT, 3.797 days after the GRB trigger. We acquired a series of 8 × 300 s images in the Rc and Ic filters.
The optical counterpart, also reported as AT2026fgk (Konno et al., GCN #43974; Hinds et al., GCN #43977; Lipunov et al., GCN #43978; Martin-Carrillo et al., GCN #43979; Moreno Méndez et al., GCN #43980; de Ugarte Postigo et al., GCN #43984; Hsu et al., GCN #43986; Pursiainen et al., GCN #43990; Becerra et al., GCN #43991; Martin-Carrillo et al., GCN #44000; Watson et al., GCN #44001; Lai et al., GCN #44002) is clearly detected in the individual frames.
In the Rc-band stacked image, we measure a preliminary magnitude of Rc = 18.21 +/- 0.07 mag (AB), calibrated against nearby stars from the Pan-STARRS catalog and appropriately transformed to Rc magnitudes. The measured magnitude may still be affected by contamination from the bright host galaxy, but further analysis is ongoing.
GCN Circular 44002
Subject
GRB 260310A / AT2026fgk: Multiband optical follow-up with Kinder observations
Date
2026-03-14T08:19:29Z (6 months ago)
From
Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com>
Via
Web form
C.-H. Lai, A. Aryan, Y.-H. Lee, C.-S. Lin, T.-W. Chen (all NCU), A. K. H. Kong (NTHU), J. H. Gillanders, S. J. Smartt (both Oxford), Y. J. Yang (NYUAD), A. Sankar.K, M.-H. Lee, Y.-C. Pan, C.-C. Ngeow, W.-J. Hou, H.-Y. Hsiao, H.-C. Lin, J.-K. Guo (all NCU), S. Yang, Z. N. Wang, L. L. Fan, G. H. Sun (all HNAS), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, T. Moore, K. W. Smith, C. Angus, A. Aamer (all QUB), A. Schultz, and M. Huber (both IfA, Hawaii) report:
We performed multiband optical observations of the field of the GRB 260310A (Fermi GBM team, GCN 43951, Hamburg et al., GCN 43975) using the 1m LOT at the Lulin observatory, as part of the Kinder collaboration (Chen et al. 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first LOT epoch of observations in g-band started at 13:45 UTC on 13th of March 2026 (MJD 61092.577), 3.37 days after the Fermi GBM detection.
The reported optical counterpart, AT 2026fgk (Konno et al., GCN 43974; Hinds et al., GCN 43977; Lipunov et al., GCN 43978; Martin-Carrillo et al., GCN 43979; Moreno Méndez et al., GCN 43980; Ugarte Postigo et al., GCN 43984; Hsu et al., GCN 43986; Pursiainen et al., GCN 43990; Becerra et al., GCN 43991; Martin-Carrillo et al., GCN 44000; Watson et al., GCN 44001), is clearly visible in each individual frame. The GRB also had X-ray (Salunke et al., GCN 43958; Jayaraman et al., GCN 43994) and NIR (Li et al., GCN 43993; Stein et al., GCN 43996) detections. We utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtractions with the Pan-STARRS1 3Pi archive images (Chambers et al. 2016 arXiv:1612.05560) using the 'SFFT' (Hu et al. 2022, ApJ, 936, 157) algorithm.
Moreover, we further used AutoPhOT to perform PSF photometry. The details of the observations and the measured photometry (in the AB system) are as follows:
Telescope | Filter | MJD (start) | t-t0 (d) | Exposure (s) | Magnitude | avg.Seeing | Airmass
LOT | g | 61112.573 | 3.367 | 300 * 1 | 18.72 +/- 0.02 | 1".94 | 2.35
LOT | r | 61112.576 | 3.371 | 300 * 1 | 18.42 +/- 0.02 | 1".70 | 2.32
LOT | i | 61112.581 | 3.374 | 300 * 1 | 18.26 +/- 0.02 | 1".77 | 2.28
LOT | z | 61112.584 | 3.378 | 300 * 1 | 17.96 +/- 0.05 | 1".90 | 2.25
In multiband photometry, the optical counterpart appears red.
The presented magnitudes are calibrated using the field stars from the ATLAS-refcat2 catalog from MAST (Tonry J. L. et al. 2018, ApJ, 867, 105). In the direction of the transient, these magnitudes are not corrected for the expected Galactic foreground extinctions of 0.09, 0.06, 0.04, and 0.03 mags (Schlafly & Finkbeiner 2011) for the g, r, i, and z bands, respectively. The details on the Lulin observatory telescopes and the follow-up methodology for such transients are presented in Aryan et al. (2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69).
GCN Circular 44001
Subject
GRB 260310A / AT 2026fgk: COLIBRÍ continuing optical observations
Date
2026-03-14T06:20:02Z (6 months ago)
From
Alan Watson at UNAM <alan@astro.unam.mx>
Via
Web form
Alan M. Watson (UNAM), Damien Dornic (CPPM), Rosa L. Becerra (UNAM), Antonio de Ugarte Postigo (LAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Camila Angulo (UNAM), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), and Benjamin Schneider (LAM) report:
We again imaged the field of AT 2026fgk (Lipunov et al., GCN Circ. 43954; Konno et al., GCN Circ. 43975; Hinds et al., GCN Circ. 43977; Lipunov et al., GCN Circ. 43978, Martin-Carrillo et al., GCN Circ. 43079; Moren Méndez et al., GCN Circ 43980; de Ugarte Postiga et al., GCN Circ. 43984; Hsu et al., GCN Circ. 43986; Pursiainen et al., GCN Circ. 43990; Becerra et al., GCN Circ 43991; Li et al., GCn Circ. 43993; Stein et al., GCN Circ 43996; Martin-Carrillo et al., GCn Circ. 44000), a possible counterpart of the Fermi GBM GRB 260310A (Fermi GBM Team, GCN Circ. 43951; Humburg & Meegan, GCN Circ. 43975) and also detected by AstroSat CZTI (Salunke et al., GCN Circ. 43958), using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-03-14 02:46 to 05:07 UTC (from 93.82 to 96.17 hours after the GRB trigger) and obtained 15, 14, and 28 minutes, respectively, of exposure in the g, r, and z filters.
The data were reduced, coadded, calibrated, and analysed with the ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.
In the stacked images, we continue to detect AT 2026fgk with a preliminary PSF magnitude of:
r = 18.47 +/- 0.01
Compared to the magnitudes in our first epoch reported in Moreno Méndez et al. (GCN Circ. 43980), we estimate a temporal decay index of about -1.2, consistent with the normal decay observed in GRBs. Moreover, from the optical/IR magnitudes from our first epoch and the photometry reported with WINTER (Stein et al., GCN Circ. 43996) between T+71.5 and T+72.5 hours, we estimate a spectral index beta of about -1.1 also supporting a synchrotron spectrum and a likely origin in a GRB afterglow.
We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM.
COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.
GCN Circular 44000
Subject
GRB 260310A / AT2026fgk: further CAHA 1.23m telescope optical observations suggest shallow decay
Date
2026-03-14T04:43:17Z (6 months ago)
From
Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>
Via
Web form
A. Martin-Carrillo, E. Aasen, L. Brown, D. Colfer, M. Collins, J. Costello, C. Harris, R. Odunuga, H. O'Leary Jeffers, C. O'Malley, J. Parimkayala, J. Roca Garcia-Valino, R. Roddy, D. Scott, A. Valdivia y Alvarado Carvajal (UCD), A. Fernandez-Martin (CAHA), J. F. Agui Fernandez (CAHA), report on behalf of a larger collaboration:
We observed the position of AT2026fgk, the likely counterpart to GRB 260310A (Fermi GBM team, GCN 43951; Salunke et al., GCN 43958; Konno et al., GCN 43974; Hinds et al., GCN 43977; Lipunov et al., GCN 43978; Martin-Carrillo et al., GCN 43979; Méndez et al., GCN 43980; de Ugarte Postigo et al., GCN 43984; Hsu et al., GCN 43986; Malesani et al., GCN 43990; Becerra et al., GCN 43991, Li et al., GCN 43993; Stein et al., GCN 43996; discovered by GOTO; O'Neill et al. 2026, TNS Discovery Report 294132) using the CAHA 1.23m telescope equipped with the DLR-MkIII CCD camera. A series of 3 x 150s exposures were taken in the BVRI bands starting at 2026-03-13T23:59:00 (~3.793 days after the Fermi trigger).
The candidate is still well detected in the individual images with a preliminary AB magnitude of R = 18.3 +/- 0.1. Our photometry was calibrated using nearby stars from the Pan-STARRS catalogue and is neither corrected for Galactic extinction nor the host galaxy contribution.
Comparing this new epoch with our previously reported R-band magnitude (Martin-Carrillo et al., GCN 43979) we infer a shallow decay with a power-law index of ~0.3, significantly shallower than the decay observed (~0.7) since the initial report by Konno et al., GCN 43974.
These observations were made during the UCD Academy observing run at Calar Alto Observatory (Almeria, Spain) as part of the final year undergraduate projects of the University College Dublin BSc (Hons) in Physics with Astronomy and Space Science.
GCN Circular 43996
Subject
GRB 260310A/AT 2026fgk: WINTER NIR Detections
Date
2026-03-13T21:05:01Z (6 months ago)
From
Robert David Stein at JSI <rdstein@umd.edu>
Via
Web form
Robert Stein (UMD), Geoffrey Mo (Caltech/Carnegie), Viraj Karambelkar (Columbia), Xander J. Hall (CMU), Tomas Ahumada (NOIRLab), Danielle Frostig (CfA), Mansi Kasliwal (Caltech), Nathan Lourie (MIT), Benjamin Schneider (LAM), Robert Simcoe (MIT), and report:
We observed AT 2026fgk (Konno et al., GCN 43974; Hinds et al., AstroNote 2026-65; O’Neill et al. 2026, TNS Discovery Report 294132), identified as the likely counterpart to GRB 260310A (Fermi GBM Team, GCN 43951). We used the Palomar 1-m telescope, equipped with the 1.2-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024).
Observations began at 2026-03-12T09:57:09.115 UTC (~53 hours after the GRB trigger), and continued over several expochs in both J-band and Hs-band. The images were processed using the WINTER data reduction pipeline implemented with mirar (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565).
We clearly detect a source in all of our stacked images, with times given relative to burst time (2026-03-10T04:57:11):
| t - t0 (hr) | filter | mag(AB) | magerr | limiting mag |
|-------------|--------|---------|--------|--------------|
| 53.00 | J | 17.16 | 0.09 | 19.28 |
| 53.58 | J | 17.28 | 0.09 | 19.18 |
| 54.13 | Hs | 17.10 | 0.21 | 17.91 |
| 71.87 | J | 17.34 | 0.12 | 19.03 |
| 72.17 | Hs | 16.97 | 0.20 | 17.77 |
We do not perform image subtraction due to a lack of suitable reference images, so there might be some contamination from the host galaxy. However, given the large offset and relatively faint host, we do not expect this to substantially alter our measurements.
We do not measure significant evolution over our ~20 hour baseline, after accounting for the uncertainty in each measurement. A slow fading would be consistent with these observations, and with expectations for a power-law decay of an afterglow at this relatively late (>2 days) phase.
Our observations are consistent with values reported by Li et al (GCN 43993), who quote a value of J=16.7 +/- 0.1 mag in the Vega system (~17.6 mag AB) at a later phase of +81 hours.
Further observations of this transient are planned.
WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research.
GCN Circular 43994
Subject
GRB 260310A: Einstein Probe Follow-up X-ray Telescope (FXT) Detection
Date
2026-03-13T17:39:34Z (6 months ago)
From
Rahul Jayaraman at Cornell University <rj438@cornell.edu>
Via
Web form
Rahul Jayaraman (Cornell), Anna Y. Q. Ho (Cornell), K-Ryan Hinds (Caltech), S. Y. Fu (HUST), R. D. Liang, M. J. Liu, Z. X. Ling, Hui Sun, Weimin Yuan (NAO, CAS), Genevieve Schroeder (Cornell), Xander J. Hall (CMU), Michael Coughlin (UMN) report:
The Follow-up X-ray Telescope on board the Einstein Probe observed the position of AT2026fgk, the likely counterpart to GRB 260310A (Fermi GBM team, GCN 43951; Arya et al., GCN 43958; Konno et al., GCN 43974; Martin-Carrillo et al., GCN 43979; Méndez et al., GCN 43980; Becerra et al., GCN 43991), beginning at 2026-03-12T17:22:58 UTC. The exposure time of this observation was 1498 s. An uncatalogued X-ray source was detected at RA = 219.3158, DEC = 71.8417, with an uncertainty of 10 arcsec (radius, 90% CL statistical and systematic); this X-ray source is consistent with the optical position initially reported by Hinds et al., AstroNote 2026-65.
We performed a preliminary analysis of the FXT spectrum in the 0.5-10 keV energy range from both the A and B modules. We fit an absorbed power-law with a line-of-sight hydrogen column density of 2.65 x 10^20 cm^-2. We find a photon index of 1.55 ± 0.09, with an unabsorbed flux of (5.51 ± 0.22) x 10^-12 erg/s/cm^2. At the redshift of z = 0.153 from Ugarte Postigo et al. (GCN 43984), this would correspond to an X-ray luminosity of (3.74 ± 0.15) x 10^44 erg/s. Parameter uncertainties are reported at the 90% C.L.
We consider this source the likely X-ray afterglow to GRB 260310A, and encourage further follow-up.
The Einstein Probe is a space mission supported by the Strategic Priority Program on Space Sciénce of the Chinese Academy of Sciences, in collaboration with the European Space Agency, the Max-Planck-Institute for Extraterrestrial Physics (Germany), and the Centre National d'études Spatiales (France).
GCN Circular 43993
Subject
GRB 260310A/AT 2026fgk: J-band observations by the SYSU 80 cm infrared telescope
Date
2026-03-13T17:35:21Z (6 months ago)
Edited On
2026-03-14T19:29:31Z (6 months ago)
From
lijj328@mail2.sysu.edu.cn
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of lijj328@mail2.sysu.edu.cn
Via
Web form
Jin-Ji Li, Chun Chen, Duo-Le Cao, Zhong-Nan Dong, Wei-Sen Huang, Jia-Qi Lin, Pu Lin, Yun Shi, Hao-Nan Yang, Yan Yu, P H Thomas Tam, Rong-Feng Shen, Bin Ma (Sun Yat-sen University) report on behalf of the SYSU 80cm infrared telescope team:
We observed the field of GRB 260310A (Fermi GBM Team, GCN 43951; Salunke et al., GCN 43958), using the Sun Yat-sen University (SYSU) 80 cm infrared telescope. Our observations were carried out on 2026 Mar 13, 13:58 UT, 81.0 hours after the GRB trigger, using stacked images in the J band.
We clearly detect the source AT 2026fgk, the likely counterpart to GRB 260310A (Konno et al., GCN 43974; Hinds et al., GCN 43977; Lipunov et al., GCN 43978; Martin-Carrillo et al., GCN 43979; Moreno Méndez et al., GCN 43980; Atteia et al., GCN 43981; de Ugarte Postigo et al., GCN 43984; Hsu et al., GCN 43986; Malesani et al., GCN 43990; Becerra et al., GCN 43991; discovered by GOTO; O'Neill et al. 2026, TNS Discovery Report 294132), and we measure J = 16.7 ± 0.1 Vega mag. The photometry was calibrated against nearby stars from the 2MASS catalog and is reported in the Vega system, without correction for Galactic extinction.
The SYSU 80 cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University.
GCN Circular 43991
Subject
GRB 260310A/AT 2026fgk: DDOTI Optical Detection
Date
2026-03-13T14:38:23Z (6 months ago)
From
Rosa Leticia Becerra Godínez at Instituto de Astronomía, UNAM <rbecerra@astro.unam.mx>
Via
Web form
Rosa L. Becerra (UNAM), Alan M. Watson (UNAM), Camila Angulo Valdez (UNAM), Nat Butler (ASU), Simone Dichiara (Penn State University), Tsvetelina Dimitrova (ASU), Alexander Kutyrev (GSFC/UMD), William H. Lee (UNAM), Océlotl López (UNAM), Margarita Pereyra (UNAM) and Eleonora Troja (U Roma) report:
We observe the field of the Fermi/GBM GRB 260310A (Fermi GBM Team, GCN 43951) with the DDOTI/OAN wide-field imager at the Observatorio Astronómico Nacional on Sierra San Pedro Mártir (http://ddoti.astroscu.unam.mx) on the nights of 2026-03-10 and 2026-03-11 UTC. DDOTI observed from 05:11 UTC to 9:34 on 2026-03-10 (from T+0.2 hours to T+4.6 hours after the trigger) with a total exposure time of 2.1 hours and from 03:32 UTC to 12:20 on 2026-03-11 (from T+22.6 hours to T+31.4 hours after the trigger), with a total exposure time of 3.1 hours.
On both nights we detect the source AT2026fgk (discovered by GOTO; O’Neill et al. 2026, TNS Discovery Report 294132), proposed as the optical counterpart (Konno et al.; GCN Circ. 43974), with preliminary AB magnitudes of:
w ≈ 16.6 (in our first epoch);
w = 17.52 +/- 0.01 (in our second epoch).
The photometry of the second night is consistent with a temporal decay of alpha≈-0.6 between the magnitudes reported by the LAST collaboration (Konno et al.; GCN Circ. 43974) and COLIBRÍ (Moreno Méndez et al.; GCN Circ. 43980).
Due to technical problems during the first night, the photometry from that epoch should be considered tentative.
These values are not corrected for the Galactic extinction.
We thank the staff of the Observatorio Astronómico Nacional on the Sierra of San Pedro Mártir.
GCN Circular 43990
Subject
GRB 260310A: NOT optical polarimetry
Date
2026-03-13T14:33:29Z (6 months ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
M. Pursiainen (Warwick), D. B. Malesani (DAWN/NBI and Radboud), G. Leloudas (DTU Space), Dimple (Birmingham), A. Martin-Carrillo (UCD), B. P. Gompertz (Birmingham), D. A. Perley (LJMU), N. Pyykkinen (NOT and UTU), report on behalf of a larger collaboration:
We observed the likely optical counterpart AT 2026fgk (Konno et al., GCN 43974; Hinds et al., AstroNote 2026-65; O’Neill et al. 2026, TNS Discovery Report 294132) of GRB 260310A (Hamburg & Meegan, GCN 43975; Salunke et al., GCN 43958), using the Nordic Optical Telescope (NOT) equipped with ALFOSC, carrying out imaging polarimetry.
In an image taken on 2026 Mar 13.09 UT (2.89 days after trigger), the transient had an AB magnitude of r = 18.12 +- 0.02 (calibrated against Pan-STARRS sources, not corrected for Galactic extinction). Unfortunately the seeing conditions were poor (~2.8" FWHM).
A sequence of 4x900 s exposures was secured in linear imaging polarimetry (mean epoch Mar 13.09 UT), employing four different retarder plate angles and an SDSS r filter. The data were reduced with a custom-built reduction pipeline (Pursiainen et al. 2025, doi:10.1093/mnras/staf232) based on the photutils python library (Bradley et al. 2024, doi:10.5281/zenodo.13989456), with reduction steps detailed in Pursiainen et al. (2023, doi:10.1051/0004-6361/202345945).
No significant polarization is found for the transient, down to a 3-sigma upper limit of P < 1.5%. No correction for Galactic interstellar polarization has been applied, though we note that the line of sight to GRB 260310A only suffers from very modest extinction (A_V = 0.05 mag).
GCN Circular 43986
Subject
GRB 260310A/AT 2026fgk: Bok spectroscopic observation
Date
2026-03-13T11:30:56Z (6 months ago)
Edited On
2026-03-13T15:31:14Z (6 months ago)
From
Brian Hsu at University of Arizona <bhsu@arizona.edu>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Brian Hsu at University of Arizona <bhsu@arizona.edu>
Via
Web form
B. Hsu (U of Arizona), M. Shrestha (Monash U), Jennifer Andrews (Gemini-N/NOIRLab), D. J. Sand (U of Arizona), N. Franz (U of Arizona), J. Pearson (U of Arizona), C. Christy (U of Arizona), C. L. Ransome (U of Arizona), Bhagya Subrayan (U of Arizona), K. Bostroem (U of Arizona), G. Hosseinzadeh (UCSD), N. Smith (U of Arizona)
We observed the possible optical counterpart to GRB 20260310A/AT2026fgk (Konno et al., GCN 43974; Lipunov et al., GCN 43978, discovered by GOTO; O'Neill et al. 2026, TNS Discovery Report 294132) on UT 2025-03-13T06:02:23.850 using the B&C spectrograph on the 90-inch Bok telescope.
The optical counterpart is clearly visible. The optical spectrum shows a featureless red continuum, although we find a strong, narrow emission line at 7566 Angstroms that we attribute to H-alpha from the candidate host galaxy at z = 0.153, which is in agreement with the redshift obtained by Hinds et al., GCN 43977. Additional follow-up observations are encouraged.
GCN Circular 43984
Subject
GRB 260310A / AT2026fgk: OSIRIS+/GTC spectroscopy confirms redshift z = 0.153
Date
2026-03-13T11:16:28Z (6 months ago)
From
Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com>
Via
email
A. de Ugarte Postigo (LAM), L. Izzo (INAF/OACN and DARK/NBI), A. Martin-Carrillo (UCD), D. B. Malesani (DAWN/NBI and Radboud), S. Geier (GTC), C. C. Thoene (AbAO), M. A. Aloy (UV), J. P. U. Fynbo (DAWN/NBI), L. Galbany (IEEC-CSIC), G. Lombardi (GTC), N. A. Rakotondrainibe (LAM), B. Schneider (LAM), N. R. Tanvir (Univ. Leicester), D. A. Perley (LJMU), F. M. Pérez Toledo (GTC), D. Pérez Valladares (GTC) report:
We observed AT2026fgk (a.k.a. GOTO26buh, O’Neill et al. 2026, TNS Discovery Report 294132; Hinds et al., AstroNote 2026-65), proposed as the optical counterpart (Konno et al., GCN 43974; Hinds et al., GCN 43977; Lipunov et al., GCN 43978; Martin-Carrillo et al., GCN 43979; Moreno Méndez et al., GCN 43979) of GRB 260310A (Hamburg & Meegan, GCN 43975; Salunke et al., GCN 43958) using the Gran Telescopio Canarias (GTC) equipped with the OSIRIS+ instrument. The seeing conditions were very poor (3.5" measured in the acquisition image).
In the 30-s acquisition image (beginning on 2026-03-13 at 04:23:43 UT, that is 2.98 days after trigger), the optical afterglow is well detected with a magnitude r = 18.05 + 0.05 (AB), calibrated against nearby Pan-STARRS objects, and not corrected for Galactic extinction. We warn that, given the poor seeing, some contamination from the nearby galaxy cannot be excluded.
A total of 3x600 s spectra were secured in each of grism R1000B (3600-7800 AA) and R1000R (5100-10000 AA), with mean epoch 2026-03-13 at 04:58:13 UT (3.00 days after trigger). The slit covers both the optical transient and the core of the nearby galaxy.
Continuum is visible over the entire range 3600-10000 AA. We detect a number of emission features that extend from the galaxy core to the location of the transient, with a typical drift due to galaxy rotation. These features include [O II], [O III], H-beta, H-alpha, [N II] and [SII] at a redshift of z = 0.153, confirming, and consolidating, the measurement by Hinds et al. (GCN 43977). No absorption features are detected either at the emission redshift (no Ca II, Ca I or Na I) nor in any other part of the spectrum. The lack of Mg II detection (typically the strongest feature in GRB afterglow spectra) in our spectral range would discard a redshift larger than 0.3 for a long GRB sight line with typical line strengths (using as reference the sample of de Ugarte Postigo et al. 2012, doi:10.1051/0004-6361/201219894). In particular at a redshift of 0.3, where the SNR would be worse, Mg II would have to be fainter than 95% of the GRB sight lines. At z = 0.7, this line of sight would need to be weaker than any in the sample. Furthermore, the chance probability of a transient appearing within 5" from the galaxy without being related is just ~1% (Moreno Méndez et al., GCN 43979). All of this indicates that the transient is indeed at the redshift of z = 0.153.
The GRB afterglow is located in the outskirts of a galaxy, at a projected distance of ~13.5 kpc from the galaxy core. This offset is very large for a GRB from a collapsar origin and more in line with the expectations for mergers (e.g. Fong et al. 2022, doi: 10.3847/1538-4357/ac91d0), although not unprecedented even for collapsars (see e.g. Thoene et al. 2024, doi:10.1051/0004-6361/202348141). Together with the location in the merger region of the Epeak - Eiso diagram (Atteia et al., GCN 43981), this opens the possibility that GRB 260310A had a merger progenitor. Further monitoring to constrain the emergence of an associated SN will clarify the nature of the source and is strongly encouraged.
This work has used the GRBspec database at http://grbspec.eu (de Ugarte Postigo et al. 2014, doi:10.1117/12.2055774).
GCN Circular 43981
Subject
GRB 260310A / AT 2026fgk: Location in the Epeak - Eiso diagram
Date
2026-03-13T08:04:42Z (6 months ago)
From
Jean-Luc Atteia at IRAP <jean-luc.atteia@irap.omp.eu>
Via
Web form
Jean-Luc Atteia (IRAP), Enrique Moreno Méndez (UNAM), Massimiliano Lincetto (CPPM), Damien Dornic (CPPM), Rosa L. Becerra (UNAM), Alan M. Watson (UNAM), Camila Angulo (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), and Antonio de Ugarte Postigo (LAM) report:
we computed the isotropic equivalent energy of GRB 260310A (Fermi GBM Team, GCN Circ. 43951) , using the spectral information on the prompt emission measured by Fermi GBM (Hamburg & Meegan, GCN Circ. 43975), in order to locate the GRB in the Amati diagram.
At z = 0.153 (Hinds et al. GCN 43977), Ep,i = 198 keV, Eiso = 3.5e50 erg and GRB 260310A clearly lies in the Type I (mergers) region of the Amati plot, which is unusual for a long GRB.
In order to locate GRB 260310A in the Type II (collapsars) region of the Amati plot, it has to be at z ≥ 0.7, and not associated with the bright galaxy noticed by the LAST team (Konno et al., GCN Circ. 43974).
GCN Circular 43980
Subject
GRB 260310A / AT 2026fgk: COLIBRÍ optical observations
Date
2026-03-13T04:01:22Z (6 months ago)
Edited On
2026-03-15T22:19:00Z (6 months ago)
From
enriquemm@astro.unam.mx
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of enriquemm@astro.unam.mx
Via
Web form
Enrique Moreno Méndez (UNAM), Massimiliano Lincetto (CPPM), Damien Dornic (CPPM), Rosa L. Becerra (UNAM), Alan M. Watson (UNAM), Camila Angulo (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), and Antonio de Ugarte Postigo (LAM) report:
We imaged the field of AT 2026fgk (first reported by Hinds et al. in AstroNote 2026-65) and followed-up by several groups (Lipunov et al., GCN Circ. 43954; Konno et al., GCN Circ. 43975; Hinds et al., GCN Circ. 43977; Lipunov et al., GCN Circ. 43978, Martin-Carrillo et al., GCN Circ. 43079), a possible counterpart of the Fermi GBM GRB 260310A (Fermi GBM Team, GCN Circ. 43951; Humburg & Meegan, GCN Circ. 43975) and also detected by AstroSat CZTI (Salunke et al., GCN Circ. 43958), using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-03-13 02:37 to 03:06 UTC (from 69.67 to 70.15 hours after the GRB trigger) and obtained 8, 8, 8, 8, and 16 minutes, respectively, of exposure in the g, r, i, z, and y filters.
The data were reduced, coadded, calibrated, and analysed with the ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.
In the stacked images, we detect AT 2026fgk with preliminary PSF magnitudes of:
g = 18.53 +/- 0.01
r = 18.15 +/- 0.01
i = 17.95 +/- 0.01
z = 17.74 +/- 0.01
y = 17.63 +/- 0.02
Our photometry is consistent with the reported by Martin-Carrillo (GCN Circ. 43979) and with the colours expected for a low-redshift GRB, supporting its association with GRB 260310A (Fermi GBM Team, GCN Circ. 43951).
As was firstly noticed by the LAST team (Konno et al.; GCN Circ. 43974), this source is projected separation of 11.3 kpc from the center of a nearby galaxy with photo-z ≈ 0.11 (Legacy Survey DR10, Dey et al. 2019) and likely spectroscopic redshift z = 0.153 (Hinds et al., GCN Circ. 43977). This source has a probability of chance alignment with the nearby galaxy of <1%, strongly suggesting an association.
We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams.
COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.
GCN Circular 43979
Subject
GRB 260310A / AT2026fgk: CAHA 1.23m telescope optical observations
Date
2026-03-13T02:28:03Z (6 months ago)
From
Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>
Via
Web form
A. Martin-Carrillo, J. Roca Garcia-Valino, E. Aasen, L. Brown, D. Colfer, M. Collins, J. Costello, C. Harris, R. Odunuga, H. O'Leary Jeffers, C. O'Malley, J. Parimkayala, R. Roddy, D. Scott, A. Valdivia y Alvarado Carvajal (UCD), J. F. Agui Fernandez (CAHA), A. Fernandez-Martin (CAHA) report on behalf of a larger collaboration:
We observed the optical counterpart candidate (Konno et al., GCN 43974; Lipunov et al. GCN 43978, discovered by GOTO; O'Neill et al. 2026, TNS Discovery Report 294132) of GRB 260310A (Fermi GBM Team, GCN 43951; Salunke et al. 2026, GCN 43958) using the CAHA 1.23m telescope equipped with the DLR-MkIII CCD camera. A series of 3 x 150s exposures were taken in the BVR bands starting at 2026-03-12T22:57:09 (~2.75 days after the Fermi trigger).
The candidate is well detected in the individual images with a preliminary AB magnitude of R = 18.2 +/- 0.1. Our photometry was calibrated using nearby stars from the Pan-STARRS catalogue and is not corrected for Galactic extinction. It should be noted that it is likely that this value has some contribution from the nearby likely host galaxy.
These observations were made during the UCD Academy observing run at Calar Alto Observatory (Almeria, Spain) as part of the final year undergraduate projects of the University College Dublin BSc (Hons) in Physics with Astronomy and Space Science.
GCN Circular 43978
Subject
GRB 260310A / AT2026fgk : MASTER OT rebrightening during 7h observations
Date
2026-03-13T02:18:22Z (6 months ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
V.Lipunov (Lomonosov MSU), N.Budnev, O.Gress (ISU),
A.Kuznetsov, I.Panchenko, P.Balanutsa, G.Antipov, E.Gorbovskoy, N.Tiurina, Ya.Kechin, V.Senik, A.Chasovnikov, M.Shilova,V.Topolev, D.Vlasenko, A.Sankovich, Yu.Tselik (Lomonosov MSU),
D.Svinkin (Ioffe Institute),
M.J. Segura, C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A.Sosnovskij (CrAO),
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,A.R.Corella, J.Tanori, L. Villalobos, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory)
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)
MASTER Global Robotic Net [1-4] started Fermi GRB 260310A observations (Fermi team GCN 43951)
at MASTER-Tunka robotic telescope at 2026-03-10 16:01:36 UT (Lipunov et al. GCN 43954)
Optical transient AT2026fgk, discovered by GOTO at 2026-03-10 05:14:30.624 (TNSC 1001, https://www.wis-tns.org/object/2026fgk )
was detected as MASTER OT J143716.19+715031.2 at our images at 2026-03-12 13:46:11UT with m_unfiltered~18.3.
This OT was observed at 400 images with rebrigtening detection to 17.2 at 2026-03-12 21:02:59 UT.
Observation and reduction will continue.
[1] Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L
[2] Lipunov et al. 2022, Universe, Vol. 8(5), id.271
[3] Lipunov et a. 2019, ARep, vol.63, 293
[4] Lipunov et al. 2023, Astronomical Robotic Networks and Operative Multichanel Astrophysics,Lomonosov MSU PRESS, 591pp.
http://www.pereplet.ru/lipunov/625.html#625
GCN Circular 43977
Subject
GRB260310A / AT2026fgk: P200 Spectroscopy
Date
2026-03-13T00:27:51Z (6 months ago)
From
khinds@caltech.edu
Via
Web form
K-Ryan Hinds (Caltech), Daniel A. Perley (LJMU), Anna Y. Q. Ho (Cornell), Sam Rose (Caltech), Jillian Rastinejad (UMd), Robert Stein (UMd), Geoffrey Mo (Caltech), Mansi Kasliwal (Caltech), Jesper Sollerman (OKC), Xander J. Hall (CMU), Jacob Wise (LJMU), Rahul Jayaraman (Cornell), Aleksandra Bochenek (LJMU)
We previously reported the optical transient AT 2026fgk (ID 294132) as an afterglow candidate based on its fast evolution, red color, and high implied luminosity (Hinds et al., AstroNote 2026-65). Konno et al. (GCN 43974) reported additional detections of AT 2026fgk using the Large Array Survey Telescope (LAST) whilst tiling the probability region for GRB 260310A (Fermi GBM Team, GCN 43951; Salunke et al., GCN 43958), and noted the temporal and spatial coincidence that identifies AT 2026fgk as the likely optical counterpart of this GRB.
We observed GRB 260310A starting at 2026-03-12 10:23:42 UTC (~2.2 d after the Fermi GBM trigger time) with the Next Generation Palomar Spectrograph (NGPS; AstroNote 2024-340) on the Palomar 5.1m Hale Telescope (P200). The optical spectrum shows a featureless red continuum, although there is a strong, narrow emission line at 7566 Angstroms that we attribute to H-alpha from the candidate host galaxy at z = 0.153. If this is the redshift of the GRB, it would be among the closest long-duration bursts to date.
For GRB 260310A, using a fluence of 5.8e-6 erg/cm^2 (10-1000 keV) reported by Hamburg et al. (GCN 43795) and assuming the redshift above, we find the gamma-ray E_iso is ~3.69e50 erg, characteristic of low-luminosity long-GRBs. The peak observed optical luminosity (from ATLAS forced photometry) is M_o ~ -22.7 mag at 7.2 hours post-trigger, more luminous than the equivalent value seen in most other nearby LLGRBs.
An associated supernova similar to SN 1998bw would peak at an apparent magnitude of r = 19.7 at this redshift in the absence of host extinction. Further observations are encouraged.
GCN Circular 43975
Subject
GRB 260310A: Fermi GBM Observation
Date
2026-03-12T22:19:13Z (6 months ago)
From
rhamburg@usra.edu
Via
Web form
R. Hamburg (USRA) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:
"At 04:57:10.81 UT on 10 March 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260310A (trigger 794811435/260310206), which was also detected by AstroSat (Salunke et al. 2026, GCN 43958). This GRB has been reported to have a potential optical counterpart, AT2026fgk, which was discovered by GOTO (O'Neill et al. 2026, TNS Discovery Report 294132) and LAST (Konno et al. 2026, GCN 43974). AT2026fgk has a photometric redshift measurement of z = 0.11 (Hinds et al. 2026, AstroNote 2026-65).
The angle from the Fermi LAT boresight is 124 degrees.
The GBM light curve consists of a single emission episode with a duration (T90) of about 60 s (50-300 keV). The time-averaged spectrum from T0-9.6 to T0+61.6 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.15 +/- 0.01 and the cutoff energy, parameterized as Epeak, is 172 +/- 5 keV.
The event fluence (10-1000 keV) in this time interval is (5.8 +/- 0.4)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+9.8 s in the 10-1000 keV band is 2.3 +/- 0.3 ph/s/cm^2.
The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html
For Fermi GBM data and info, please visit the official Fermi GBM Support Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"
GCN Circular 43974
Subject
GRB 260310A: potential optical counterpart
Date
2026-03-12T19:11:47Z (6 months ago)
From
Ruslan Konno at Weizmann Institute of Science <ruslankonno@gmail.com>
Via
Web form
R. Konno (WIS), S. Garrappa (WIS), E. A. Zimmerman (WIS), A. Horowicz (WIS), E. O. Ofek (WIS), S. Ben-Ami (WIS), D. Polishook (WIS), O. Yaron (WIS), S. Fainer (WIS), A. Krassilchtchikov (WIS), Y. M. Shani (WIS), E. Segre (WIS), A. Gal-Yam (WIS), and S. Spitzer (WIS) report on behalf of the LAST Collaboration.
We report observations of GRB 260310A, detected by Fermi/GBM (Fermi GBM Team, GCN 43951), as well as by AstroSat/CZTI (Salunke et al., GCN 43958). Observations were conducted with the Large Array Survey Telescope (LAST; Ofek et al. 2023, PASP 135, 5001; Ben-Ami et al. 2023, PASP 135, 5002).
Observations of GRB 260310A were taken over several tiling epochs starting at 2026-03-10 16:37:45 UTC (T-T0 = 11.67 h), in clear band (similar to the Gaia Bp band) with 24 telescopes. Each epoch consists of 20x20s exposures per telescope. The follow-up covers an integrated localization probability of 97% as given within the HEALPix FITS file glg_healpix_all_bn260310206.fit (Fermi GBM Team, GCN 43951).
Initially, we do not detect a credible counterpart within our automated pipeline due to incomplete reference coverage of the field. However, we note AT 2026fgk (discovered by GOTO; O'Neill et al. 2026, TNS Discovery Report 294132) as a potential counterpart. The transient was first detected at T-T0 = 1040 s, has an angular separation of about 7 deg from the best-fit Fermi/GBM position, and is shown to be highly variable (Hinds et al., AstroNote 2026-65). This transient is clearly detected within the LAST data and shows an initial rise and a subsequent steady decay, consistent with Hinds et al., AstroNote 2026-65. The LAST photometry is as follows
| Tmid-T0 (h) | Mag (AB) |
|---|---|
| 14.05 | 17.18 +/- 0.04 |
| 14.17 | 17.17 +/- 0.04 |
| 14.30 | 17.09 +/- 0.04 |
| 14.41 | 17.16 +/- 0.04 |
| 14.54 | 17.10 +/- 0.04 |
| 14.66 | 17.11 +/- 0.04 |
| 17.97 | 17.23 +/- 0.04 |
| 19.01 | 17.25 +/- 0.04 |
| 20.05 | 17.19 +/- 0.04 |
| 21.09 | 17.30 +/- 0.04 |
| 22.02 | 17.32 +/- 0.04 |
Furthermore, serendipitous LAST observations of the field prior to the GRB event time, up to 2026-03-10 02:51:01 UTC (T-T0 = -2.15 h), shows no detection up to a 5-sigma limiting magnitude of 20.74 (AB). We thus note this transient as a potential optical counterpart, and encourage further follow-up observations.
LAST is a survey telescope array of the Weizmann Astrophysical Observatory (https://www.weizmann.ac.il/wao/).
GCN Circular 43958
Subject
GRB 260310A: AstroSat CZTI detection of a long burst
Date
2026-03-11T07:19:18Z (6 months ago)
From
Anuraag Arya at IIT Bombay <aryaanuraag910@gmail.com>
Via
Web form
S. Salunke (IUCAA), Harsha K. H. (IUCAA), M. Tembhurnikar (IUCAA), A. Arya (IITB), A. Goyal (IITB), G. Waratkar (Caltech/IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:
Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a long GRB 260310A which was also detected by Fermi GBM (Fermi GBM Team, GCN Circ. 43951).
The source was clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2026-03-10 04:57:12.05 UTC. The measured peak count rate associated with the burst is 273 (+67, -57) counts/s above the background in the combined data of all quadrants, with a total of 2709 (+573, -589) counts. The local mean background count rate was 1494 (+6, -6) counts/s. We measure a T90 of 30 (+2, -8) s from the cumulative Veto light curve.
The source was also clearly detected in the 20-200 keV energy range.
CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project.
CZTI GRB detections are reported regularly on the payload site at:
http://astrosat.iucaa.in/czti/?q=grb
GCN Circular 43954
Subject
Fermi GRB 260310A: Global MASTER-Net observations report
Date
2026-03-10T16:32:56Z (6 months ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko,
G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A.Sosnovskij (CrAO),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory)
MASTER-Tunka robotic telescope [1] located in Russia (Applied Physics Institute, Irkutsk State University) started inspect of the Fermi GRB 260310A ( Fermi GBM team, GCN 43951) errorbox 39832 sec after notice time and 39865 sec after trigger time at 2026-03-10 16:01:36 UT, with upper limit up to 20.0 mag. The observations began at zenith distance = 36 deg. The sun altitude is -40.5 deg.
MASTER-Kislovodsk robotic telescope located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) started inspect of the Fermi GRB 260310A errorbox 40477 sec after notice time and 40510 sec after trigger time at 2026-03-10 16:12:21 UT, with upper limit up to 18.3 mag. Observations started at twilight. The observations began at zenith distance = 51 deg. The sun altitude is -12.2 deg.
The galactic latitude b = 38 deg., longitude l = 118 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3178578
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
39895 | 2026-03-10 16:01:36 | MASTER-Tunka | (14h 43m 00.89s , +78d 55m 32.9s) | C | 60 | 19.9 |
39895 | 2026-03-10 16:01:36 | MASTER-Tunka | (14h 45m 33.78s , +78d 47m 59.9s) | C | 60 | 20.0 |
40197 | 2026-03-10 16:06:37 | MASTER-Tunka | (14h 33m 24.43s , +80d 47m 57.5s) | C | 60 | 19.1 |
40197 | 2026-03-10 16:06:37 | MASTER-Tunka | (14h 36m 13.25s , +80d 42m 38.1s) | C | 60 | 19.2 |
40540 | 2026-03-10 16:12:21 | MASTER-Kislovodsk | (13h 26m 03.01s , +78d 44m 12.4s) | C | 60 | 17.9 |
40643 | 2026-03-10 16:14:03 | MASTER-Kislovodsk | (14h 10m 06.46s , +76d 46m 16.8s) | C | 60 | 17.8 |
40749 | 2026-03-10 16:15:49 | MASTER-Kislovodsk | (14h 29m 09.94s , +78d 40m 44.7s) | C | 60 | 18.1 |
40850 | 2026-03-10 16:17:31 | MASTER-Kislovodsk | (13h 41m 13.66s , +80d 36m 24.9s) | C | 60 | 18.3 |
40941 | 2026-03-10 16:19:01 | MASTER-Kislovodsk | (13h 56m 38.66s , +74d 54m 16.8s) | C | 60 | 18.0 |
41046 | 2026-03-10 16:20:47 | MASTER-Kislovodsk | (14h 57m 06.59s , +80d 32m 41.8s) | C | 60 | 18.3 |
41150 | 2026-03-10 16:22:30 | MASTER-Kislovodsk | (15h 03m 52.45s , +76d 43m 39.0s) | C | 60 | 17.9 |
41357 | 2026-03-10 16:25:57 | MASTER-Kislovodsk | (14h 46m 01.37s , +74d 52m 17.5s) | C | 60 | 14.0 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
[1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023, Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp.
http : // www.pereplet.ru/lipunov/625.html
GCN Circular 43951
Subject
GRB 260310A: Fermi GBM Final Real-time Localization
Date
2026-03-10T05:07:50Z (6 months ago)
From
Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply@GIOC.nsstc.nasa.gov>
Via
email
The Fermi GBM team reports the detection of a likely LONG GRB
At 04:57:10 UT on 10 Mar 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260310A (trigger 794811435.80987 / 260310206).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 213.6, Dec = 78.7 (J2000 degrees, equivalent to J2000 14h 14m, 78d 42'), with a statistical uncertainty of 4.3 degrees.
The angle from the Fermi LAT boresight is 124.0 degrees.
The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260310206/quicklook/glg_skymap_all_bn260310206.png
The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260310206/quicklook/glg_healpix_all_bn260310206.fit
The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260310206/quicklook/glg_lc_medres34_bn260310206.gif