GRB 261010A
GCN Circular 45975
Subject
GRB261010A: Retraction of Keck/MOSFIRE near-IR source
Date
2026-10-17T18:13:42Z (5 days ago)
From
Jillian Rastinejad at University of Maryland <jillianrastinejad2024@u.northwestern.edu>
Via
Web form
Jillian Rastinejad (UMD), Maggie Li, Mansi Kasliwal (Caltech), and Charles D. Kilpatrick (Northwestern) report on behalf of a larger collaboration:
We have re-reduced our Keck/MOSFIRE J-band observation (Rastinejad et al. GCN 45957) of GRB 261010A (Fermi GBM Team, GCN 45888; Lien et al., GCN 45889, Frederiks et al. GCN 45925). In our updated stacked image, the reported near-IR source is no longer detected at >3-sigma significance. We therefore refine our conclusion from the Keck/MOSFIRE imaging at 5.4 days post-burst (Rastinejad et al. GCN 45957) to place a 3-sigma upper limit on emission at the position of the optical afterglow (O’Neill et al., GCN 45890) of J > 23.5 AB mag. This upper limit may be improved with ongoing data reduction.
GCN Circular 45964
Subject
GRB 261010A: Pan-STARRS z-band imaging and upper limit
Date
2026-10-16T16:13:47Z (6 days ago)
From
James Gillanders at University of Oxford <jhgillanders.astro@gmail.com>
Via
Web form
J. H. Gillanders (Oxford), M. Huber, K. C. Chambers, D. Jones, A. S. B. Schultz (IfA, Univ. Hawaii), S. J. Smartt, K. W. Smith, (Oxford/QUB), S. Srivastav (Oxford), M. Nicholl, D. Young (QUB), T.-W. Chen (NCU, Taiwan) T. de Boer, J. Fairlamb, G. Paek, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, I. A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii).
We observed the field of GRB 261010A, discovered by the Fermi GBM Team (GCN 45888) and also detected by Swift/BAT (Lien et al., GCN 45889), Konus-Wind (Frederiks et al., GCN 45925), GRID (Yu et al., GCN 45940) and Glowbug (Cheung et al., GCN 45950), with the Pan-STARRS1 telescope (Chambers et al., 2016, arXiv e-prints, 1612.05560) in an effort to constrain the presence of transient flux at the position of the afterglow, as discovered by O’Neill et al. (GCN 45890), and subsequently detected by Aceituno et al. (GCN 45891), Corcoran et al. (GCN 45912), Castro-Tirado et al. (GCN 45928), Marchesini et al. (GCN 45952) and Rastinejad et al. (GCN 45957).
Our observation commenced on MJD 61327.22 (exposure midpoint; corresponding to 3.41 days post-GRB) and consisted of 6x300s exposures in z-band. We processed our images with the standard Pan-STARRS pipeline, whereby the images were astrometrically and photometrically calibrated before stacking (Magnier et al., 2020a, ApJS, 251, 3; Magnier et al., 2020b, ApJS, 251, 6; Waters et al., 2020, ApJS, 251, 4). No difference imaging was applied as the PS1 3Pi reference stack is significantly shallower than this 1800s stack.
We inspected this stacked target image and no detection of a source at the position of the afterglow is visible. We estimate a z-band 3-sigma upper limit, m_z > 22.5. At the redshift of the purported nearby host galaxy (z = 0.070; Corcoran et al., GCN 45912), this corresponds to a limit of M_z = -15 (assuming a distance modulus of 37.5, and ignoring extinction effects). At 3.4 days post-burst, AT2017gfo possessed m_z = 18.74 +/- 0.02 (Smartt et al., 2017, Nature, 551, 75), which, rescaled to z = 0.070, corresponds to m_z ~ 23. Thus, we would not expect to recover a signal for an AT2017gfo-like kilonova at this redshift. Of course this is also the case for an association with the alternative proposed host galaxy at z = 0.785 (Castro-Tirado et al., GCN 45928).
The Pan-STARRS system consists of two 1.8m telescope units located at the summit of Haleakala on the Hawaiian island of Maui, employing an SDSS-like filter system denoted as grizy, and a broad w-filter, which is a composite of the gri-filters.
GCN Circular 45961
Subject
GRB 261010A: VLA radio observations
Date
2026-10-16T14:30:05Z (6 days ago)
From
Genevieve Schroeder at Cornell University <genevieveschroeder@u.northwestern.edu>
Via
Web form
G. Schroeder (U. Toronto), W. Fong (Northwestern), J. Rastinejad (UMD), Gokul Srinivasaragavan (Northwestern) report:
We observed the location of GRB 261010A (Fermi GBM team, GCN 45888; Lien et al., GCN 45889; Frederiks et al., GCN 45925, Cheung et al., GCN 45950) with the Karl G. Jansky Very Large Array (VLA) in D configuration under program 26A-062 (PI Schroeder) for 0.75 hour at a mean frequency of 10 GHz and mid time of 2026 Oct 15 at 23:40 UT (5.1 days post-burst).
In preliminary analysis, we do not detect any radio sources at or near the position of the afterglow (Osborne et al., GCN 45894, O’Neill et al., GCN 45890; Aceituno et al., GCN 45891; Corcoran et al., GCN 45912), to a 3-sigma limit of < 20 microJy.
At z=0.070 (Corcoran et al., GCN 45912) this limit corresponds to a rest-frame luminosity of < 2e27 erg/s/Hz. This luminosity limit is 1-2 orders of magnitude lower than the typical range for radio detected short GRBs (e.g., Anderson et al. 2025) and an order of magnitude deeper than the strongest radio limits on short GRBs. This limit is also ~4 times lower than the long merger-driven GRB 230307A (e.g. Levan et al. 2024), but similar to limits placed on the long merger-driven GRB 211211A (e.g. Rastinejad et al. 2022). At z=0.785 (Castro-Tirado et al., GCN 45928), this corresponds to a rest-frame luminosity of < 3e29 erg/s/Hz. This luminosity limit is comparable to limits placed on short GRBs with no radio detections, but is about an order of magnitude lower than the typical range for long GRBs from collapsars (e.g. Chandra and Frail 2012).
Further observations are planned. We thank the VLA staff for quickly approving and executing these observations.
GCN Circular 45957
Subject
GRB261010A: Keck/MOSFIRE detection of a near-IR source
Date
2026-10-16T06:42:59Z (7 days ago)
From
Jillian Rastinejad at University of Maryland <jillianrastinejad2024@u.northwestern.edu>
Via
Web form
Jillian Rastinejad (UMD), Maggie Li, and Mansi Kasliwal (Caltech) report on behalf of a larger collaboration:
We observed the location of GRB 261010A (Fermi GBM Team, GCN 45888; Lien et al., GCN 45889, Frederiks et al. GCN 45925) using the Multi-Object Spectrograph for Infrared Exploration (MOSFIRE) mounted on the Keck I Telescope. We obtained 73 x 40-sec imaging in J-band at a mid-time of 05:17 UT on 2026 Oct 16 (5.4 days post-burst) at seeing of ~0.7”.
We detect a near-IR source at >3 sigma significance that is consistent with the position of the optical afterglow (O’Neill et al., GCN 45890; Aceituno et al., GCN 45891; Corcoran et al., GCN 45912). Precise photometry is complicated by contamination from the nearby galaxy, but we roughly measure a J-band magnitude that is consistent with the reported LBT measurement of H = 24.0 +/- 0.3 AB mag (Marchesini et al. GCN 45952). We observe a slight offset (< 0.5”) to the North between the reported optical afterglow position (O’Neill et al., GCN 45890) and the near-IR source in our MOSFIRE imaging. Given the lack of variability, we are unable to conclude if this emission is due to the counterpart of GRB 261010A or is due to an underlying galaxy.
We thank the Keck staff, including Julie Renaud-Kim and Josh Walawender, and the Keck classical night observers, including Gillian Willson for their co-operation with the ToO observations.
GCN Circular 45953
Subject
GRB 261010A: ASKAP non-detection of a radio afterglow
Date
2026-10-16T02:10:54Z (7 days ago)
From
Dougal Dobie at University of Sydney <ddobie94@gmail.com>
Via
Web form
D. Dobie (University of Sydney/OzGrav) and J. Freeburn (UNC Chapel Hill)
We report follow-up of GRB 261010A using the Australian SKA Pathfinder (ASKAP) from 04:40-10:19 UTC on 2026-10-14.
No radio emission is detected at the position of the optical counterpart (GCN 45890, GCN 45891) with a 3-sigma upper limit of 135 uJy at 943 MHz.
The observational data is publicly available on the CSIRO ASKAP Science Data Archive under SBID 89348.
Thank you to CSIRO staff for supporting these observations.
GCN Circular 45952
Subject
GRB 261010A: LBT tentative near-infrared detection
Date
2026-10-15T14:40:26Z (7 days ago)
From
Andrea Rossi at INAF <andrea.rossi@inaf.it>
Via
Web form
E. J. Marchesini, A. Rossi (INAF-OAS), R. Brivio, P. D’Avanzo (INAF-OAB), A. Melandri (INAF-OAR), L. Izzo (INAF-OACN and DARK/NBI), G. Stratta (INAF-OAS), R. Maccary, M. Bulla (University of Ferrara) on behalf of the GRAWITA collaboration, and D. B. Malesani (DAWN/NBI and Radboud) report:
We observed the field of GRB 261010A detected by Fermi/GBM (Fermi GBM team, GCN 45888), Swift (Lien et al., GCN 45889), and Konus-Wind (Frederiks et al., GCN 45925), in H band with the LUCI1 and LUCI2 near-infrared imager and spectrograph mounted on the Large Binocular Telescope (LBT, Mt Graham, AZ, USA). Observations were obtained on 2026-20-15 at the midtime UT 01:56:00, i.e. ~4.3 days after the GRB trigger, for a total of 62 min of on-source under very good sky conditions.
At the position of the afterglow (O’Neill et al., GCN 45890; Aceituno et al., GCN 45891; Corcoran et al., GCN 45912) we tentatively detect a faint flux excess at H = 24.0 ± 0.3 AB mag, close to our detection limit. The photometry was calibrated against 2MASS field stars.
Two possible associations have been pointed out for this GRB: with the nearby galaxy LEDA 1781464 at z = 0.07 (Corcoran et al., GCN 45912) and with a red object at z = 0.785 (Castro-Tirado et al., GCN 45928). A kilonova at redshift 0.07 would be about 1 mag brighter than our detection. For example, the KN associated with GRB 211211A at z = 0.076 reached J ~ 23 and K ~ 22.5 AB at a comparable epoch (Rastinejad et al. 2022, Nature 612, 223). Similarly, with a corresponding absolute magnitude of approximately M_H ~ −14 (AB), the tentative detection would be about 1 mag fainter than the kilonova AT 2017gfo, which had an absolute magnitude of approximately M_H ~ −15 (AB) at 5 days after the merger (Villar et al. 2017 ApJL 851 L21).
While the light curve of this transient is not well constrained, the observed emission is possibly due to the GRB afterglow at z = 0.785, or in principle even to an underlying, previously undetected host galaxy or host galaxy region. Further observations are needed to establish the nature of this faint source.
We acknowledge the excellent support from the LBTO and LBT-INAF staff, particularly J. Rupert and F. Cusano, in obtaining these observations.
GCN Circular 45950
Subject
GRB 261010A: Glowbug gamma-ray detection
Date
2026-10-14T14:32:59Z (8 days ago)
From
C.C. Cheung at Naval Research Lab <Teddy.Cheung@nrl.navy.mil>
Via
Web form
C.C. Cheung, R. Woolf, M. Kerr, J.E. Grove (NRL), A. Goldstein (USRA), C.A. Wilson-Hodge, D. Kocevski (MSFC), and M.S. Briggs (UAH) report:
The Glowbug gamma-ray telescope [1,2,3], operating on the International Space Station, reports the detection of GRB 261010A, which was also detected by Fermi/GBM (GCN 45888, 45911, 45944), Swift/BAT (GCN 45889, 45918), Konus-Wind (GCN 45925, 45945), and GRID (GCN 45940).
Using an adaptive window with a resolution of 32-ms, the burst onset is determined to be 2026-10-10 19:17:01.824 with a duration of 6.1 s and a total significance of about 26.0 sigma. The detection occurred during a time of high background as Glowbug was in the northern radiation belt. The light curve comprises an initial spike followed by the brightest peak at ~T0+4.5s.
The analysis results presented here are preliminary and use a response function that lacks a detailed characterization of the surrounding passive structure of the ISS. Glowbug is a NASA-funded technology demonstrator for sensitive, low-cost gamma-ray transient telescopes developed, built, and operated by the U.S. Naval Research Laboratory (NRL) with support from the University of Alabama in Huntsville, USRA, and NASA MSFC.
[1] Grove, J.E. et al. 2020, Proc. Yamada Conf. LXXI, arXiv:2009.11959
[2] Woolf, R.S. et al. 2022, Proc. SPIE, 12181, id. 121811O
[3] Woolf, R.S. et al. 2024, Proc. SPIE, 13151, id. 1315108
Distribution Statement A: Approved for public release. Distribution is unlimited.
GCN Circular 45945
Subject
GRB 261010A: initial pulse rest-frame energetics from Konus-Wind detection
Date
2026-10-13T18:34:08Z (9 days ago)
Edited On
2026-10-14T20:30:57Z (8 days ago)
From
Dmitry Frederiks at Ioffe Institute <fred@mail.ioffe.ru>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Dmitry Frederiks at Ioffe Institute <ddfrederiks@gmail.com>
Via
email
D. Frederiks, A.Lysenko, A. Ridnaia, D. Svinkin,
A. Tsvetkova, M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:
Using the Konus-Wind spectrum of the short initial pulse of GRB 261010A
(Ep = 847(-194,+299) keV; GCN 45925) and the KW-measured fluence of this
pulse (5.91+/-1.06)E-6 erg/cm2, we estimate the pulse's rest-frame energetics
at two possible redshifts:
- assuming the redshift z= 0.070 (Corcoran et al., GCN 45912),
the isotropic energy release E_iso to (7.1 ± 1.3)x10^49 erg,
the isotropic peak luminosity L_iso to (7.8 ± 1.4)x10^49 erg/s,
and the rest-frame peak energy Ep,z to 906(-208,+320) keV.
- at the redshift z=0.785 (Castro-Tirado et al., GCN 45928),
E_iso is (1.02 ± 0.18)x10^52 erg, L_iso is (1.85 ± 0.3)x10^52 erg/s,
and Ep,z is 1512(-346,+534) keV.
With the obtained estimates, GRB 261010A is an outlier (outside 90% prediction bands)
of both 'Amati' and 'Yonetoku' relations derived for the sample of >300 long KW GRBs
with known redshifts (Tsvetkova et al., 2017; Tsvetkova et al., 2021),
however is consistent with Type I (short/hard) GRB initial pulse energetics,
plotted in the same rest-frame planes.
See http://www.ioffe.ru/LEA/GRBs/GRB261010_T69424/GRB261010A_rest_frame.pdf
This supports the hypothesis that the burst represents a merger-driven event
(Zhu and Tam, GCN 45932; Maccary et al., GCN 45944), a scenario proposed
taking into account other properties of the burst prompt emission.
All the quoted errors are estimated at the 68% confidence level.
All the presented results are preliminary.
GCN Circular 45944
Subject
GRB 261010A: a short minimum variability timescale and a distinct pulse evolution compatible with a merger origin
Date
2026-10-13T15:27:50Z (9 days ago)
From
ROMAIN LOIC MACCARY at University of Ferrara <romain.maccary@edu.unife.it>
Via
Web form
R. Maccary, C. Guidorzi, M. Bulla (University of Ferrara) A. Rossi, G. Stratta, (INAF-OAS),S. Klose (TLS Tautenburg) on behalf of a larger collaboration report:
We determined the minimum variability timescale (MVT) of GRB 261010A (Fermi GBM team, GCN 45888) from the Fermi/GBM light curve (NaI detectors 0, 1 and 3) following the prescriptions of Camisasca et al. (2023), i.e. as the minimum full width half maximum (FWHM_min) of all statistically significant pulses, and found FWHM_min = 82 (-21, +28) ms. Combined with the T90~54 s (Fermi GBM team, GCN 45911), in the T90-FWHM_min plot GRB 261010A (green square in the Figure at this link https://drive.google.com/file/d/1xVNgXHV105Zqyeva3hRCc_OcizuGEC-0/view?usp=drive_link) lies in the region populated by other long-lasting merger candidates, such as also GRB 191019A.
In addition, the waiting times show a distinct temporal evolution (see https://drive.google.com/file/d/1udGHzr9tzvQvIQNng_tFnlRK30SuZp2v/view?usp=drive_link), which has been recently identified as an indicator of merger origin (Maccary et al. 2026a,b)
This is in agreement with the prompt-based merger classification made by Zhu & Tam, GCN 45932.
In addition, assuming either a redshift of z= 0.070 (Malesani et al., GCN 45912) or z=0.785 (Castro-Tirado et al., GCN 45928), a fluence of 1.72e-5 erg cm-2 would correspond to either Eiso~2.0e+50 erg or Eiso~2.9e+52 erg. Combined with Ep~837 keV (Frederiks et al., GCN 45925), the two possible positions of GRB261010A in the Epi-Eiso plane seem to be more compatible with Type-I rather than Type-II GRBs (see Figure at this link https://drive.google.com/file/d/1_NaXnsKbUKEF0SW9yx8TgjdR4Rb7QgY9/view?usp=drive_link).
We caution that this Figure was obtained using the peak energy computed using the main pulse, the time-integrated peak energy computed on the whole burst could be lower (e.g., Fermi/GBM team, GCN 45911) which would put the point at z=0.785 slightly closer to Type-II GRBs (see https://drive.google.com/file/d/1z5MI3L9aGI8hhrtL5kHurP6my6yEtAPU/view?usp=drive_link) .
References:
- Minimum variability timescale (MVT): Camisasca et al., 2023,https://doi.org/10.1051/0004-6361/202245657
- Maccary et al., 2025,A&A,https://doi.org/10.1051/0004-6361/202555418
- Distinctive pulse evolution: Maccary et al., 2026, A&A, https://doi.org/10.1051/0004-6361/202659083
GCN Circular 45940
Subject
GRID detection of GRB 261010A
Date
2026-10-13T12:32:49Z (9 days ago)
From
GRID Student Team at Tsinghua University <grid@tsinghua.edu.cn>
Via
Web form
GRID detection of GRB 261010A
Zihan Yu, Chenyu Wang, Xinyu Liu report on behalf of the GRID Collaboration:
GRID-11B reports the detection of the long-duration GRB 261010A, which was also detected by Fermi GBM (GCN Circular 45888, 45911), Swift BAT (GCN Circular 45889, 45918), and Konus-Wind (GCN Circular 45925).
X-ray and optical afterglow detections were reported by Swift XRT (GCN Circular 45889, 45894, 45935), NOT/ALFOSC (GCN Circular 45890, 45912), and the OSN 1.5m telescope (GCN Circular 45891).
We adopt 2026-10-10 at 19:17:03 UTC, the Fermi GBM trigger time, as T0. Using a source interval from T0-2 to T0+8 s, the measured burst duration (T90) using events selected in the 30-2000 keV range of the GRID-11B FITS energy column is approximately 5.4 +/- 1.2 seconds (statistical uncertainty).
The extended emission reported by other instruments is not clearly seen in the GRID-11B light curve. The reported T90 characterizes the main emission episode.
The GRID light curve of this event can be found at https://mirrors.tuna.tsinghua.edu.cn/GRID/data/GRID-GCN/GRB261010A/GRID_261010A_ltcv.pdf .
GRID is a student-led project to monitor the transient gamma-ray sky with multiple detectors onboard different nanosatellites in the era of multi-messenger astronomy. For more information about GRID, please refer to the following references: https://doi.org/10.1007/s10686-019-09636-w and https://doi.org/10.1007/s10686-021-09819-4 .
GCN Circular 45937
Subject
GRB 261010A: J-band upper limit by the SYSU 80cm infrared telescope
Date
2026-10-13T10:41:14Z (9 days ago)
From
Jinji Li <lijj328@mail2.sysu.edu.cn>
Via
Web form
Jin-Ji Li, Si-Rui Huang, Jian-Hong Cao, Zhong-Nan Dong, Rui-Chen Gao, Jia-Qi Lin, Pu Lin, Jing-Kai Sun, Yun Shi, Hao-Nan Yang, Hao-Ran Zhang, P. H. Thomas Tam, Rong-Feng Shen, and Bin Ma (Sun Yat-sen University) report on behalf of the SYSU 80cm infrared telescope team:
We observed the field of GRB 261010A, detected by Swift (Lien et al., GCN 45889) and Fermi/GBM (Fermi GBM Team, GCN 45888), using the Sun Yat-sen University (SYSU) 80cm infrared telescope. Our J-band observations were carried out on 2026 Oct 11 from 13:25:44 to 14:24:51 UT, approximately 18.1–19.1 hours after the Swift/BAT trigger. We combined 171 exposures of 20 s each.
We do not detect a counterpart at the reported optical afterglow position (O'Neill et al., GCN 45890; Corcoran et al., GCN 45912), down to a 5-sigma limiting magnitude of J ~ 19.6 mag (Vega).
The SYSU 80cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University.
GCN Circular 45935
Subject
GRB 261010A: Swift-XRT refined Analysis
Date
2026-10-13T07:38:02Z (9 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
K.L. Page and P.A. Evans (U. Leicester) reports on behalf of the
Swift-XRT team:
We have analysed 4.7 ks of XRT data for GRB 261010A, from 4.4 ks to
21.5 ks after the trigger. The data are entirely in Photon Counting
(PC) mode.
The light curve can be modelled with a power-law decay with a decay
index of alpha=1.08 (+0.25, -0.24).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.70 (+0.37, -0.22). The
best-fitting absorption column is 4 (+11, -0) x 10^20 cm^-2,
consistent with the Galactic value of 4.6 x 10^20 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum is 3.9 x 10^-11 (4.2 x
10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 4 (+11, -0) x 10^20 cm^-2
Galactic foreground: 4.6 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index: 1.70 (+0.37, -0.22)
If the light curve continues to decay with a power-law decay index of
1.08, the count rate at T+24 hours will be 4.4 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.7 x
10^-13 (1.9 x 10^-13) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/01516528.
This circular is an official product of the Swift-XRT team.
GCN Circular 45932
Subject
GRB 261010A: a Type I-L (merger-driven) GRB candidate
Date
2026-10-13T05:42:21Z (10 days ago)
From
Thomas P.H. Tam at Sun Yat-sen University <grbtom@gmail.com>
Via
Web form
Si-Yuan Zhu and Pak-Hin Thomas Tam (SYSU) report:
We analyzed GRB 261010A using publicly available data from Fermi/GBM. The burst was detected by Fermi/GBM at 19:17:03 UT on 2026 October 10, as a long-duration GRB with T90 about 54 s (50-300 keV; Fermi GBM Team, GCN 45888).
The 10-1000 keV light curve of GRB 261010A consists of multiple emission episodes. In particular, we identified a precursor emission (PE) episode from T0-0.128 s to T0+0.384 s, with a duration of T100,PE = 0.512 s. The PE is separated from the main emission (ME, from T0 + 3.856 s to T0 + 5.936 s), by a quiescent interval (QE) lasting T100, QE=3.472 s. We adopted the pulse rise time of the PE as its minimum variability timescale (MVT), T_MVT,PE = 0.152 s. The time-integrated spectrum of the ME is best fitted by a power law with an exponential cutoff, yielding a photon index of alpha = -0.94 +/- 0.04 and a peak energy of Ep,ME=970 +/- 198 keV.
To further classify GRB 261010A, we apply the Ep,ME-precursor index (EPI), a diagnostic that uses prompt emission alone (Zhu et al. 2026; [1]). The EPI separates merger-driven (Type I) from collapsar-driven (Type II) GRBs, with Type I GRBs concentrating at EPI > 6.2 and Type II GRBs at EPI < 6.2. Values in the range 6.0 < EPI < 6.4 fall in a transition region where misclassification is possible, whereas EPI > 6.4 and EPI < 6.0 allow safe classification as Type I and Type II, respectively. We calculate EPI = 6.81, suggesting that GRB 261010A is unambiguously classified as a Type I-L (merger-driven) GRB. (see https://tinyurl.com/epi-grb261010a)
The optical afterglow of GRB 261010A (O'Neill et al., GCN 45890; Aceituno et al., GCN 45891) is located at a projected offset of 34.5 kpc from the nearby galaxy LEDA 1781464 at z = 0.070 (Corcoran et al., GCN 45912). GTC/OSIRIS spectroscopy of the red object 2.2 arcsec NE of the afterglow shows a galaxy at z = 0.785 +/- 0.001 (Castro-Tirado et al., GCN 45928), and the afterglow is offset from the centre of that galaxy by 16 kpc in projection.
If LEDA 1781464 at z = 0.070 is the host, the large projected offset of 34.5 kpc supports a merger origin. We obtain Eiso = 5.04E49 erg and Ep,z = 1038 keV of the ME, consistent with the Ep,z-Eiso correlation of Type I (short) bursts. If the galaxy at z = 0.785 is the host. We obtain Eiso = 7.19E51 erg and Ep,z = 1731 keV of the ME, still consistent with the Type I correlation.
Based on our preliminary analysis, GRB 261010A is a Type I-L (merger-driven) GRB candidate. Further analysis of GRB 261010A is ongoing and multi-wavelength follow-up observations are strongly encouraged.
References [1] Zhu et al., ApJ, 1003, 142, 2026
GCN Circular 45928
Subject
GRB 261010A: 10.4m GTC spectroscopy of the galaxy 2" from the afterglow
Date
2026-10-12T21:17:07Z (10 days ago)
From
Alberto Castro-Tirado at Inst.de Astro. de Andalucia <ajct@iaa.es>
Via
email
A. J. Castro-Tirado, I. Pérez-García (IAA-CSIC), Y.-D. Hu (GXU), F. J. Aceituno, M.D. Caballero-Garcia, E. Fernandez-Garcia, M. Gritsevich, S. Guziy (IAA-CSIC), on behalf of a larger collaboration, report:
We observed the field of GRB 261010A, detected by Fermi/GBM (the Fermi team, GCN 45888), Swift (Lien et al. GCN 45889), and Konus-Wind (Frederiks et al. GCN 45925), with the OSIRIS+ spectrograph at the 10.4m Gran Telescopio Canarias in La Palma (Spain). Long-slit spectra with the R1000R and R1000B grisms (600 s each) were taken on Oct 11, 2026, starting at 20:28 UT, i.e. 25.2 h post burst. The slit covered both the position of the optical afterglow (O’Neill et al., GCN 45890; Aceituno et al., GCN 45891) and the red object located 2.2 arcsec NE of it (Corcoran et al., GCN 45912).
The spectrum of the redder object confirms the source to be a galaxy, displaying the [O II] 3727 A emission line, the Ca II H&K absorption doublet and the 4000 A break, all consistent at a redshift z = 0.785 +/- 0.001.
At this redshift the projected offset of the afterglow from the galaxy centre is 16 kpc.
If this galaxy at 2” is the host, GRB 261010A would have an isotropic-equivalent energy of E_iso ~ 3-4 x 10^52 erg and a rest-frame peak energy of ~0.8-1.5 MeV (using the Fermi/GBM and Konus-Wind spectral parameters, GCN 45911 and 45925), well above the Amati relation of long GRBs, thus placing this event in the same parameter space as GRB 211211A and GRB 230307A (both long GRBs which originated from a compact binary merger rather than a collapsar).
Further observations are planned.
We thank the GTC staff for the rapid execution of these observations.
GCN Circular 45925
Subject
Konus-Wind detection of GRB 261010A
Date
2026-10-12T16:48:04Z (10 days ago)
From
Dmitry Frederiks at Ioffe Institute <fred@mail.ioffe.ru>
Via
email
D. Frederiks, A.Lysenko, A. Ridnaia, D. Svinkin,
A. Tsvetkova, M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:
The long-duration GRB 261010A (Swift detection:
Lien, et al., GCN 45889; Barthelmy et al., GCN 45918;
Fermi-GBM detection: The Fermi GBM team, GCN 45888;
Palafox & Jameson, GCN 45911)
triggered Konus-Wind (KW) at T0=69424.522 s UT (19:17:04.522).
The KW triggered on a bright, hard-spectrum, multipeaked pulse,
with a duration of ~3 s, which is superimposed over a broad,
less intense emission pulse that started ~10 s before the trigger,
peaked at ~T0+16 s, and lasted for ~70 s.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB261010_T69424/
As observed by Konus-Wind, the burst had
a fluence of (1.72 ± 0.47)x10^-5 erg/cm^2 and
a 64-ms peak energy flux, measured from T0 + 0.128 s,
of (6.02 ± 1.08)x10^-6 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).
The spectrum of the bright pulse (measured from T0 to T0+8.448 s)
is best fitted in the 20 keV - 15 MeV range by a power law with exponential
cutoff (CPL) model dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = -0.98 (-0.12, + 0.15) and Ep = 837(-194,+299) keV, PGstat = 81/98 dof.
Fitting this spectrum by a Band GRB function yields the same values
of alpha and Ep, and only an upper limit on the high energy photon
index beta of -2.0
The spectrum of the broad pulse (measured from T0+8.448 s to T0+49.408 s)
is best fitted in the 20 keV - 15 MeV range by a simple PL function
with the PL index of 1.92 (-0.07,+0.08),
PGstat = 103/99 dof.
All the quoted errors are estimated at the 68% confidence level.
All the presented results are preliminary.
GCN Circular 45918
Subject
GRB 261010A: Swift-BAT refined analysis
Date
2026-10-12T04:10:48Z (11 days ago)
From
Amy <yarleen@gmail.com>
Via
email
S. D. Barthelmy (GSFC), R. Gupta (GSFC), H. A. Krimm (NSF), S. Laha
(GSFC/UMBC), A. Y. Lien (U Tampa), C. B. Markwardt (GSFC), M. J. Moss
(GSFC), D. M. Palmer (LANL), T. Parsotan (GSFC), D. Sadaula (GSFC/UMBC), T.
Sakamoto (AGU) (i.e. the Swift-BAT team):
Using the data set from T-120 to T+662 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 261010A (trigger #1516528)
(Lien et al., GCN Circ. 45889). The BAT ground-calculated position is
RA, Dec = 248.767, 26.600 deg which is
RA(J2000) = 16h 35m 04.2s
Dec(J2000) = +26d 36' 01.3"
with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment). The
partial coding was 83%.
The mask-weighted light curve shows a main pulse that starts and peaks at
~T0 and ends at ~T+3 s. In addition, there are some weak emission that
starts prior to the main pulse at ~T-10 s and lasts till ~T+70 s. T90
(15-350 keV) is 71.50 +- 14.07 sec (estimated error including systematics).
The time-averaged spectrum from T-9.28 to T+76.45 sec is best fit by a
simple power-law model. The power law index of the time-averaged spectrum
is 1.43 +- 0.17. The fluence in the 15-150 keV band is 3.1 +- 0.4 x 10^-6
erg/cm2. The 1-sec peak photon flux measured from T+0.26 sec in the 15-150
keV band is 3.6 +- 0.4 ph/cm2/sec. All the quoted errors are at the 90%
confidence level.
The results of the batgrbproduct analysis are available at
https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1516528
(In memory of Scott Barthelmy, the BAT team dedicates the rest of our
circulars to Scott.)
GCN Circular 45912
Subject
GRB 261010A: NOT confirmation of the optical afterglow
Date
2026-10-11T22:16:41Z (11 days ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), D. O’Neill (Birmingham), L. Izzo (INAF/OAC and DARK/NBI), B. P. Gompertz (Birmingham), A. Martin-Carrillo (UCD), A. Schenone-Zanuzzi (NOT), E. Georgiou (NOT and Turku Univ.) report on behalf of a larger collaboration:
We observed again the position of the Swift and Fermi GRB 261010A (Lien et al., GCN 45889; Palafox & Jameson, GCN 45911) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC imager. A total of 6x300 s were secured in the SDSS r band, with a mean epoch 2026 Oct 11.85154 UT (25.152 hr after trigger).
The candidate optical counterpart (O’Neill et al., GCN 45890; Aceituno et al., GCN 45891) is no longer detected in our images. Performing image subtraction using the Legacy survey as template, we measure a limiting magnitude r > 23.9 (AB; calibrated against nearby Pan-STARRS objects, and not corrected for Galactic extinction).
We also note that the refined Swift/XRT position (Osborne et al., GCN 45894) has slightly shifted compared to the promptly-released coordinates (Lien et al., GCN 45889), and is now fully consistent with the location of the optical counterpart candidate. We thus conclude that this object is indeed the afterglow of GRB 261010A.
The position of the afterglow is about 25" (34.5 kpc in projection) away from the nearby galaxy LEDA 1781464 at redshift z = 0.070. The chance association probability is of order 1% (comparable to that for the fainter, redder object 2" away from the afterglow; O’Neill et al., GCN 45890). We note that, if placed at z = 0.070, our limit implies a luminosity > 1.5 mag fainter than AT 2017gfo at a comparable phase (e.g. Villar et al. 2017, doi:10.3847/2041-8213/aa9c84).
GCN Circular 45911
Subject
GRB 261010A: Fermi GBM Observation
Date
2026-10-11T21:47:48Z (11 days ago)
Edited On
2026-10-12T01:56:32Z (11 days ago)
From
Eva MP at INAOE <eva.palafox@gmail.com>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Eva MP at INAOE <eva.palafox@gmail.com>
Via
Web form
E. Palafox (INAOE) and A. Jameson (USRA) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:
"At 19:17:03.16 UT on 10 October 2026, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 261010A (trigger 813352628/261010804).
which was also detected by Swift BAT (A. Y. Lien et al. 2026, GCN 45889).
The Fermi GBM on-ground location is consistent with the Swift BAT position.
The angle from the Fermi LAT boresight is 22 degrees.
The GBM light curve consists of a bright emission episode follwed by several lower peaks with a duration (T90)
of about 54 s (50-300 keV). The time-averaged spectrum
from T0-7.4 to T0+54.0 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -1.30 +/- 0.05 and the cutoff energy,
parameterized as Epeak, is 450 +/- 90 keV.
The event fluence (10-1000 keV) in this time interval is
(1.70 +/- 0.05)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+4.2 s in the 10-1000 keV band is 8.2 +/- 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 45894
Subject
GRB 261010A: Enhanced Swift-XRT position
Date
2026-10-11T10:24:10Z (11 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
J.P. Osborne, A.P. Beardmore, P.A. Evans and M.R. Goad (U. Leicester)
report on behalf of the Swift-XRT team.
Using 821 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 261010A, we find an astrometrically corrected X-ray
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 248.74803, +26.58216 which is equivalent
to:
RA (J2000): 16h 34m 59.53s
Dec (J2000): +26d 34' 55.8"
with an uncertainty of 3.0 arcsec (radius, 90% confidence).
This position may be improved as more data are received. The latest
position can be viewed at https://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).
This circular was automatically generated, and is an official product of the
Swift-XRT team.
GCN Circular 45891
Subject
GRB 261010A: possible optical afterglow detected with the 1.5m OSN
Date
2026-10-10T21:42:39Z (12 days ago)
From
Alberto Castro-Tirado at Inst.de Astro. de Andalucia <ajct@iaa.es>
Via
email
F.J. Aceituno, E. J. Fernandez-Garcia, A. J. Castro-Tirado, I. Perez-Garcia, M.D. Caballero-Garcia, M. Gritsevich, S. Guziy (IAA-CSIC) and Y.-D. Hu (GXI), on behalf of a larger collaboration, report:
Following the detection of GRB 261010A by both Fermi (Fermi team, GCN 45888) and Swift/BAT (Lien et al. GCN 45889), we triggered the 1.5m telescope at Sierra Nevada Observatory (OSN) in Granada (Spain), starting on Oct 10, 19:49 UT (~32 min post trigger) in the VRI filters. On the coadded image (7x300s, I-band), an uncatalogued object is detected slightly outside the Swift/XRT error circle, on the outskirts of a galaxy visible in the DSS2 archival plate. For the transient we measure a preliminary magnitude of I = 21.0 mag, at coordinates (J2000) = 16:34:59.45 +26:34:57.5, in agreement with the NOT findings (O’Neill et al. GCN 45890). Further observations are needed to confirm its relation to GRB 261010A.
We thank the staff at OSN for their excellent support.
GCN Circular 45890
Subject
GRB 261010A: NOT possible optical counterpart
Date
2026-10-10T21:39:17Z (12 days ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
D. O’Neill (Birmingham), B. Schneider (LAM), D. B. Malesani (DAWN/NBI and Radboud), B. Gompertz (Birmingham), L. Izzo (INAF/OACN and DARK/NBI), A. Martin-Carrillo (UCD), N. R. Tanvir (Leicester), A. Schenone-Zanuzzi (NOT), report on behalf of a larger collaboration:
We observed the position of the Swift and Fermi GRB 261010A (Lien et al., GCN 45889; Fermi GBM team, GCN 45888) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations started as soon as weather conditions cleared, on 2026 Oct 10.874 UT (1.7 hr after the GRB trigger).
Both by visual inspection and by digital image subtraction (versus Pan-STARRS and Legacy), we note the presence of a source not seen in archival imaging, with the following coordinates (J2000):
RA = 16:34:59.46
Dec = +26:34:57.4
This position is slightly (~2") offset from a red source visible in the Legacy Survey images; the relation between these two objects is unclear. After template subtraction, we measure r ~ 21.36 +- 0.06 (AB) for the new source (not corrected for Galactic extinction).
The above coordinates are ~7" off the centre of the currently best available Swift/XRT position, which has a quoted uncertainty of 3.6". The association of this object with GRB 261010A is therefore tentative at the moment, and awaits confirmation. Further observations are planned and encouraged.
GCN Circular 45889
Subject
GRB 261010A: Swift detection of a burst
Date
2026-10-10T19:34:53Z (12 days ago)
From
Jimmy DeLaunay at PSU <jjd330@psu.edu>
Via
email
A. Y. Lien (U Tampa), R. Gupta (NASA GSFC), N. P. M. Kuin (UCL-MSSL),
M. J. Moss (GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL) and
M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:
At 19:17:07 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 261010A (trigger=1516528). Swift slewed immediately to the burst.
The BAT on-board calculated location is
RA, Dec = 248.752, 26.609 deg, which is
RA(J2000) = 16h 35m 00s
Dec(J2000) = 26d 36' 33"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve showed a single-peaked
structure with a duration of at least 5 sec. The peak count rate
was ~7000 counts/sec (15-350 keV), at ~0 sec after the trigger.
There may be additional pulse structures that are caused either
by the burst or by entry into the SAA. Additional ground data are
required to distinguish the burst emission from background noise.
The XRT began observing the field at 19:18:41.4 UT, 94.2 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 248.7491, 26.5802 which is equivalent to:
RA(J2000) = 16h 34m 59.78s
Dec(J2000) = +26d 34' 48.7"
with an uncertainty of 5.0 arcseconds (radius, 90% containment). This
location is 104 arcseconds from the BAT onboard position, within the
BAT error circle. No event data are yet available to determine the
column density using X-ray spectroscopy.
UVOT data are unavailable at this time
Burst Advocate for this burst is A. Y. Lien (yarleen AT gmail.com).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)
GCN Circular 45888
Subject
GRB 261010A: Fermi GBM Final Real-time Localization
Date
2026-10-10T19:27:41Z (12 days 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 19:17:03 UT on 10 Oct 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 261010A (trigger 813352628.162212 / 261010804).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 244.7, Dec = 27.6 (J2000 degrees, equivalent to J2000 16h 18m, 27d 36'), with a statistical uncertainty of 3.3 degrees.
The angle from the Fermi LAT boresight is 25.0 degrees.
The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn261010804/quicklook/glg_skymap_all_bn261010804.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/bn261010804/quicklook/glg_healpix_all_bn261010804.fit
The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn261010804/quicklook/glg_lc_medres34_bn261010804.gif