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GRB 260421B

GCN Circular 44665

Subject
GRB 260421B: radio detection with the VLA
Date
2026-05-19T13:17:22Z (16 days 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 01:30:46 UT on 2026 April 24 (T_mid = 2.92 days post-burst) 
and at 01:38:23 UT on 2026 April 30 (T_mid = 8.92 days post-burst)
the Karl G. Jansky VLA observed the field of GRB 260421B
(Fermi GBM team, GCN 44360, 44382; SVOM mission team, GCN 44362,
GCN 44389, 44397; Insight-HXMT team, GCN 44401) at central
frequencies of 6, 10 and 15 GHz.

The standard 3C286 was used as bandpass and flux density
calibrator, while J1239-1023 was used as phase calibrator.

From a preliminary analysis, an unresolved radio source
is detected at 8.92 days post-burst at a position (J2000):

RA: 12:40:00.478 +- 0.002
Dec: -17:44:24.70 +- 0.03

consistent with the X-ray (SVOM mission team, GCN 44362;
Einstein Probe team, GCN 44387) and optical (He et al., GCN 44363;
Sánchez Álvarez et al., GCN 44364; SVOM mission team, GCN 44365,
44377; Wu et al., GCN 44366; Pérez-Fournon et al., GCN 44383;
GRANDMA/Kilonova-Catcher collaboration, GCN 44402) 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]
================================================================
2.92	6	-	8	0.72x0.28	-38
2.92	10	-	9	0.50x0.19	-43
2.92	15	-	12	0.41x0.11	-43
8.92	6	52	8	0.58x0.27	-33
8.92	10	81	9	0.40x0.21	-38
8.92	15	137	9	0.32x0.12	-38
================================================================


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 44476

Subject
GRB 260421B: radio observation with the VLA
Date
2026-05-05T15:50:44Z (a month 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 05:08:16 UT on 2026 April 22 (T_mid = 1.07 days post-burst) 
the Karl G. Jansky VLA observed the field of GRB 260421B
(Fermi GBM team, GCN 44360, 44382; SVOM mission team, GCN 44362,
GCN 44389, 44397; Insight-HXMT team, GCN 44401) at central
frequencies of 6, 10 and 15 GHz.

The standard 3C286 was used as bandpass and flux density
calibrator, while J1239-1023 was used as phase calibrator.

From a preliminary analysis, we do not detect the radio
counterpart of GRB 260421B. We derive the following upper
limits (UL; 3sigma):

================================================================
T_mid	Freq	UL	r.m.s.	Beam		PA
[days]	[GHz]	[uJy]	[uJy/b]	[arcsec^2]	[deg]
================================================================
1.07	6	24	8	0.47x0.28	11
1.07	10	24	8	0.30x0.18	11
1.07	15	27	9	0.20x0.11	-2
================================================================

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 44402

Subject
GRB 260421B: Kilonova-Catcher optical afterglow detection
Date
2026-04-23T15:46:39Z (a month ago)
From
Damien Turpin at CEA-Saclay <dturpin-astro@hotmail.com>
Via
Web form

M. Freeberg, R. Hellot, M. Serrau (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 260421B detected by Fermi/GBM (Fermi GBM team, GCN 44360

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) and SVOM (Zhao et al., GCN 44362; Guo et al., GCN 44389) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with a CDK17 telescope located at AITP San Pedro Chile Observatory operated by R. Hellot and the TEC160FL telescope operated by M. Freeberg. Our observations started at TGRB+1.9 hours and were taken with sdss-r filter.

In our stacked frames, subtracted from the Legacy Survey DR10 template image, we marginally detect the optical counterpart (He et al., GCN 44363

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; Álvarez et al., GCN 44364; Turpin et al., GCN 44365; Wu et al., GCN 44366; Lipunov et al., GCN 44367; Schneider et al., GCN 44368; Li et al., GCN 44377; Pérez-Fournon et al., GCN 44383; Mo et al., GCN 44384; and Shi et al., GCN 44388, Pawar et al., GCN 44396).

We report our follow-up results in the table below:

Tmid-TGRB (hrs)Exp (s)FilterMagnitudeInstrument
2.4820 x 180sr (AB)21.12 +/- 0.30TEC160FL
3.879 x 300sr (AB)20.94 (3 sig U.L.)CDK17

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 44401

Subject
GRB 260421B: Insight-HXMT/HE detection
Date
2026-04-23T14:06:52Z (a month ago)
From
guohx@ihep.ac.cn
Via
Web form
Hao-Xuan Guo, Chen-Wei Wang, Cheng-Kui Li, Shao-Lin Xiong, and Chao Zheng (IHEP) report on behalf of the Insight-HXMT team:

At 2026-04-21T04:13:23.400 (T0), Insight-HXMT/HE detected a long burst, GRB 260421B, which is also detected by Fermi/GBM (Fermi GBM Team, GCN #44360), SVOM/ECLAIRs (D.Zhao et al., GCN #44362) and SVOM/GRM (Hao-Xuan Guo et al., GCN #44389).

The Insight-HXMT/HE light curve mainly consists of multipe pulses with a total duration (T90) of 16.0 +3.0/-1.5 s. The 1s peak rate, measured from T0+12.75 s, is 3246 cnts/sec. Insight-HXMT/HE detected a total of 11625 counts from this burst.

The Insight-HXMT /HE light curve can be found here:
https://www.bursthub.cn//admin/static/hxmtgrb260421B.png

All measurements above are made with the CsI detectors of Insight-HXMT/HE operating in the regular mode with the energy range of about 60-900 keV (deposited energy). Only gamma-rays with energy greater than about 200 keV can penetrate the spacecraft and leave signals in the CsI detectors installed inside of the telescope.

Insight-HXMT is the first Chinese space X-ray telescope, which was funded jointly by the China National Space Administration (CNSA) and the Chinese Academy of Sciences (CAS). More information about it could be found at: http://hxmten.ihep.ac.cn/

GCN Circular 44397

Subject
GRB 260421B: SVOM/ECLAIRs refined analysis
Date
2026-04-23T10:06:59Z (a month ago)
Edited On
2026-04-23T15:36:19Z (a month ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
M. Brunet (IRAP), L. Delfosse (CEA), O. Godet (IRAP), S. Schanne (CEA), W. L. Zhang (PMO),  Z. Y. Liu (GZNU)  report on behalf of the SVOM/ECLAIRs team

Using the event-by-event data downloaded through the X-band ground station, we report further analysis of SVOM/ECLAIRs observations of GRB 260421B (SVOM burst-id sb26042105 – GCN 44362, trigger time T0 = 2026-04-21T04:13:25 UTC), which was also detected by Fermi (GCN 44360, 44361, 44382) and SVOM/GRM (GCN 44389). 

The burst that triggered ECLAIRs onboard shows a multiple peak lightcurve. The burst duration is T90 = 26.3 +4.4/-6.0 s in the 4-120 keV energy band.

The time-averaged spectrum from T0-12 s to T0+20 s in the energy range 5-120 keV is best fitted (chi²/dof = 20.1/16) by a broken powerlaw model with best-fit parameters: alpha = -1.00 +0.35/-0.11, beta = -1.42 +0.07/-0.06 and a break energy of 13.7 +2.7/-4.4 keV. With this model, the 4-120 keV fluence is (8.6 +0.1/-1.1)e-6 erg/cm^2 and the 4-120 keV photon flux is 6.4 +0.1/-0.7 ph/cm²/s.

Using a simple powerlaw provides a bad fit (chi²/dof = 31.3/18).

The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC.

The SVOM/ECLAIRs point of contact for this burst is: Marius Brunet (IRAP) (marius.brunet at utoulouse.fr)

GCN Circular 44396

Subject
GRB 260421B: 3.6m DOT optical observation
Date
2026-04-23T10:00:19Z (a month ago)
From
pawarpankaj1492@gmail.com
Via
Web form
Pankaj Pawar, Anshika Gupta, Debalina Kar, Dhruv Jain and Kuntal Misra (ARIES) report:

We observed the field of GRB 260421B (Fermi GBM team, GCN 44360; Zhao et al., GCN 44362) using the 3.6m Devasthal Optical Telescope (DOT) located at Devasthal of the Aryabhatta Research Institute of observational sciencES (ARIES), India.

The observations began on 2026-04-21 at 21:24:53 UT, approximately 17 hours after the trigger time. We obtained 2*300s frames in the SDSS r filter. We do not detect the optical counterpart (He et al., GCN 44363; Álvarez et al., GCN 44364; Turpin et al., GCN 44365; Wu et al., GCN 44366; Lipunov et al., GCN 44367; Schneider et al., GCN 44368; Li et al., GCN 44377; Pérez-Fournon et al., GCN 44383; Mo et al., GCN 44384; and Shi et al., GCN 44388) within the error circle, nor do we see a source at the reported location of the transient.

The limiting magnitude in the stacked image is:
r > 22.4 mag (AB)

The magnitude is calibrated against the Pan-STARRS PS1 catalog.

We thank the staff of Devasthal for their assistance during these observations.

GCN Circular 44389

Subject
GRB 260421B: SVOM/GRM observation
Date
2026-04-22T08:31:00Z (a month ago)
From
guohx@ihep.ac.cn
Via
Web form
SVOM/GRM team: Hao-Xuan Guo, Chen-Wei Wang, Chao Zheng, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)

SVOM/ECLAIRs team: Olivier GODET (IRAP), Frédéric Daigne (IAP)

Report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by GRB 260421B (SVOM trigger reference: sb26042105) at 2026-04-21T04:13:22.000 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM Team, GCN #44360) and SVOM/ECLAIRs (D.Zhao et al., GCN #44362).

With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of multiple pulses with a T90 of 21.5 +3.5/-2.5 s in the 15-5000 keV band.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb260421B.png

In addition, the position of this burst, as determined by SVOM/ECLAIRs (RA = 190.0318, Dec = -17.7566, GCN #44362), is located at about 44 degrees from the SVOM optical axis, which is inside the ECLAIRs field of view.

With this localization, the time-averaged spectrum from T0-14.5 to T0+21.5 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.26 +0.07/-0.06 and the cutoff energy, parameterized as Epeak, is 162 +18/-14 keV. The event fluence (10-1000 keV) in this time interval is (1.19 +/-0.05)E-05 erg/cm^2.

The 1s peak spectrum, measured from T0+14.0 to T0+15.0 s, if fitted by a power law function with an exponential high-energy cutoff, the power law index is -0.72 +/-0.07 and the cutoff energy, parameterized as Epeak, is 200 +13/-12 keV. The flux (10-1000 keV) in this time interval is (2.27 +0.09/-0.08)E-06 erg/cm^2/s.

With the measured redshift z=2.115 (B. Schneider et al., GCN 44368), we calculate the isotropic energy Eiso is about 1.5E53 erg. Thus GRB 260421B is well consistent with Type II GRBs in the 'Amati' relation diagram, as shown at:
https://www.bursthub.cn//admin/static/svgrb260421B_amati.png
The localization of GRB 260421B in the 'Yonetoku' relation diagram is shown at:
https://www.bursthub.cn//admin/static/svgrb260421B_yonetoku.png

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. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS.

The SVOM point of contact for this burst is: Hao-Xuan Guo (IHEP)(guohx@ihep.ac.cn)

GCN Circular 44388

Subject
GRB260421B: J band upper limit by SYSU 80cm infrared telescope
Date
2026-04-22T07:24:57Z (a month ago)
From
shiy228@mail2.sysu.edu.cn
Via
Web form

Yun Shi, Chun Chen, Duo-Le Cao, Zhong-Nan Dong, Rui-Chen Gao, Wei-Sen Huang, Jia-Qi Lin, Pu Lin, Jin-Ji Li, Hao-Nan Yang, Yan Yu, Hao-Ran Zhang, 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 the long GRB 260421B, detected by Fermi-GBM (Fermi GBM team, GCN 44360

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) and SVOM/ECLAIRs (Zhao et al., GCN 44362), and followed up by Einstein Probe/FXT (Wang et al., GCN 44387), using the Sun Yat-sen University (SYSU) 80 cm infrared telescope. Early near-infrared follow-up observations have also been reported (Mo et al., GCN 44384).

Our observations were carried out from 2026 Apr 21 15:25 UT to Apr 21 17:06 UT, corresponding to 11.2–12.9 hours after the GBM trigger (2026-04-21 04:13:21 UT), with a total exposure time of 5200 s in the J band, obtained under poor conditions.

We do not detect any near-infrared counterpart at the position of the reported afterglow (He et al., GCN 44363

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; Sanchez-Alvarez, GCN 44364; Turpin et al., GCN 44365; Wu et al., GCN 44366; Li et al., GCN 44377; Pérez-Fournon et al., GCN 44383; Wang et al., GCN 44387). The 5-sigma limiting magnitude is approximately J ~ 16.9 Vega magnitudes.

The SYSU 80 cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University.

GCN Circular 44387

Subject
GRB 260421B: EP-FXT follow-up observation
Date
2026-04-22T02:42:41Z (a month ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
B. T. Wang (YNAO, CAS), J. P. Chen (SYSU), D. Y. Li, and W. D. Zhang (NAO, CAS) on behalf of the Einstein Probe (EP) team:

EP-FXT performed a follow-up observation of the SVOM/ECLAIRs-detected burst GRB 260421B (SVOM/sb26042105; Zhao et al., GCN 44362), also triggered by Fermi-GBM (Fermi GBM team, GCN 44360; Preis & Greiner, GCN 44361; Mukherjee et al., GCN 44382), starting at 2026-04-21T06:08:31UTC, approximately 1.9 hours after the SVOM trigger, with a total exposure time of 5994 s. One uncataloged source is detected within the ECLAIRs error circle, which is spatially consistent with the optical counterpart (He et al., GCN 44363; Sanchez-Alvarez, GCN 44364; Turpin et al., GCN 44365; Wu et al., GCN 44366, Li et al., GCN 44377; Pérez-Fournon et al., GCN 44383) at a redshift of z=2.115 (Schneider et al., GCN 44368). Preliminary analysis on this source is automatically conducted, and the details are listed as follows. 

EPF_J124000.4-174426
RA (J2000): 190.0018
Dec (J2000): -17.74055
Flux: 1.26e-11 erg/s/cm2 (observed, 0.5-10 keV)
Flux_err: 6.27e-13 erg/s/cm2 (1 sigma)

Launched on January 9, 2024, EP 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).

GCN Circular 44384

Subject
GRB 260421B: J-band upper limit with WINTER
Date
2026-04-21T23:32:26Z (a month ago)
From
Geoffrey Mo at Caltech / Carnegie Observatories <gmo@mit.edu>
Via
Web form

Geoffrey Mo (Caltech/Carnegie), Benjamin Schneider (LAM), Tomas Ahumada (NOIRLab), Robert Stein (UMD), Viraj Karambelkar (Columbia), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:

We observed the field of GRB 260421B (Fermi GBM Team, GCN 44360

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; Preis et al., GCN 44361; Zhao et al., GCN 44362; Mukherjee et al., GCN 44382) in the near-infrared J band with 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 2026-04-21T04:36:00 UTC in the J band (~23 min after the GRB trigger) under poor conditions, consisting of 10 x 120 s exposures. 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

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).

We do not detect a source at the optical counterpart location (He et al., GCN 44363

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; Álvarez et al., GCN 44364; Turpin et al., GCN 44365; Wu et al., GCN 44366; Lipunov et al., GCN 44367; Schneider et al., GCN 44368; Li et al., GCN 44377; Pérez-Fournon et al., GCN 44383). We obtain the following 5-sigma upper limit: J = 16.4 mag (AB).

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 44383

Subject
GRB 260421B: LCO optical counterpart detection
Date
2026-04-21T23:18:19Z (a month ago)
From
Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es>
Via
Web form
I. Pérez-Fournon (IAC and ULL), F. Poidevin (IAC and ULL), D. Cano-Morales, A.E. Hernández-Díaz, 
I. Correa-Plasencia, E. Lekaroz-Urriza, M. Quintana-Ansaldo (all ULL), A. López-Oramas (IAC and ULL), and D. Aguado (IAC and ULL)

Following the detection of the long GRB 260421B by Fermi GBM (Fermi GBM team, GCN  circ. 44360; Preis & Greiner, GCN circ. 44361; and Mukherjee, GCN circ. 44382) and by SVOM ECLAIRs, GBM, and MXT (Zhao et al., GCN circ. 44362), we observed the field with one of the two Las Cumbres Observatory (LCO) 1-m telescopes equipped with Sinistro cameras located at the LCO node at Siding Spring Observatory near Coonabarabran, New South Wales, Australia. The observation, a single exposure of 200 sec in the SDSS r' filter, started on 2026-04-21 at 13:40:22 UT, about 9.45 hours after the Fermi trigger. The optical counterpart, first reported by He et al. (GCN circ. 44363), is detected in our image  with an AB magnitude of  21.55 +/- 0.21, calibrated against PanSTARRS-1 DR2 stars and not corrected for Galactic extinction. Other optical detections or upper limits have been reported by Sánchez Álvarez et al. (GCN circ. 44364), Turpin et  al. (GCN circ. 44365), Wu et al. (GCN circ. 44366), Lipunov et al. (GCN circ. 44367), Schneider et al. (GCN circ. 44368, redshift of z = 2.115), and Li et al. (GCN circ. 44377).
 
This work makes use of observations from the Las Cumbres Observatory global telescope network (LCO program IAC2026A-011, SGLF and Superluminous Supernovae surveys).

This work made use of the Astro-COLIBRI platform (P. Reichherzer et al. 2021, ApJS, 256, 5).

GCN Circular 44382

Subject
GRB 260421B: Fermi GBM Observation
Date
2026-04-21T19:36:16Z (a month ago)
From
oindabimukherjee@gmail.com
Via
Web form
O. Mukherjee (USRA) reports on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 04:13:21.17 UT on 21 April 2026, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 260421B (trigger 798437606/260421176),
which was also detected by SVOM/ECLAIRs (Zhao et al. 2026, GCN 44362),
and VLT/X-shooter with spectroscopic redshift z = 2.115 (Schneider et al. 2026, GCN 44368).
The Fermi GBM on-ground location (GCN 44360) is consistent with the SVOM/ECLAIRs position.

The angle from the Fermi LAT boresight is 58 degrees.

The GBM light curve consists of a single emission episode with a duration (T90)
of about 18 s (50-300 keV). The time-averaged spectrum
from T0-1.6 to T0+24.9 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -1.11 +/- 0.01 and the cutoff energy,
parameterized as Epeak, is 159 +/- 1 keV.

The event fluence (10-1000 keV) in this time interval is
(1.06 +/- 0.01)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+15 s in the 10-1000 keV band is 15.4 +/- 0.3 ph/s/cm^2.

A Band function fits the spectrum equally well
with Epeak= 130 +/- 1 keV, alpha = -1 +/- 0.005 and beta = -2.31 +/- 0.02.

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 44377

Subject
GRB 260421B: SVOM/VT optical observations
Date
2026-04-21T11:53:36Z (a month ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H. L. Li, C. Wu, L. P. Xin, Y. L. Qiu, Y. N. Ma, Z. H. Yao, J. R. Xu,  X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA/Irfu), W. L. Zhang (PMO), Z. Y. Liu (GZNU) report on behalf of the SVOM/VT team.

SVOM/VT performed observations with automatic slew to the field of GRB 260421B triggered by SVOM/ECLAIRs (sb26042105, Zhao et al., GCN 44362). This burst was also detected by SVOM/GRM (Zhao et al., GCN 44362), SVOM/MXT (Zhao et al., GCN 44362), and Fermi-GBM (Fermi GBM team, GCN 44360; Preis & Greiner, GCN 44361). The observation started at 2026-04-21T04:18:32 UTC, i.e., approximately 5.1 minutes post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously. 

The optical counterpart (He et al., GCN 44363; Sanchez-Alvarez, GCN 44364; Turpin et al., GCN 44365; Wu et al., GCN 44366) with redshift z=2.115 (Schneider et al., GCN 44368) was clearly detected in single frames in both channels. With X-band data available，the light curves showed a fast decay from 5.5 minutes to 50 minutes post trigger, following a plateau after 3.37 hours.  

The preliminary measurements are given in the AB magnitudes and are not corrected for Galactic extinction:

Mid time | Band | Exposure Time |    Brightness
5.53 min   VT_B      50 sec       16.23+/-0.03 mag   
5.53 min   VT_R      50 sec       15.77+/-0.03 mag  
3.599 hr   VT_B     27*100 sec    20.94+/-0.07 mag
3.668 hr   VT_R     22*100 sec    20.52+/-0.06 mag

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 44368

Subject
GRB 260421B: VLT/X-shooter spectroscopic redshift z = 2.115
Date
2026-04-21T06:41:33Z (a month ago)
From
Benjamin Schneider at MIT <bschn@mit.edu>
Via
Web form
B. Schneider (LAM), A. Saccardi (CEA/Irfu), N. Habeeb (Leicester), R. Brivio (INAF/OAB), A. L. Thakur (INAF/IAPS), D. B. Malesani (DAWN/NBI and Radboud), report on behalf of the Stargate collaboration:

We observed the optical counterpart (He et al., GCN 44363; Sanchez-Alvarez, GCN 44364; Turpin et al., GCN 44365; Wu et al., GCN 44366) of the SVOM and Fermi GRB 260421B (Zhao et al., GCN 44362; Fermi GBM Team, GCN 44360) using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. Our spectra cover the wavelength range 3000-25000 AA, and consist of 4 exposures of 600 s each. The observation mid time was 05:39 UT on 2026 April 21 (1.42 hr after the SVOM/ECLAIRs trigger).

In a 5-s acquisition image (taken 61 min after the SVOM/ECLAIRs trigger), the afterglow is detected with a magnitude r = 20.43 +/- 0.05 AB, calibrated against the single Pan-STARRS object lying in our field of view, and not corrected for Galactic extinction.

In a preliminary reduction of the spectra, we detect a continuum over the entire wavelength range. At the blue edge of the spectrum, we identify a broad trough at around 3786 AA, likely due to H I Lyman-alpha. Multiple absorption lines are also detected, which we interpret as due to, among others, S II, Si II, O I, Si II*, C II, C II*, Si IV, C IV, Fe II, Fe II*, Mg II, all at a common redshift z = 2.115. We consider this to be the redshift of GRB 260421B.

We acknowledge the excellent expert support from the observing staff in Paranal, in particular Francisco Caceres and Matias Jones. The analysis of this spectrum was carried out with the help of the zHunter tool (https://doi.org/10.5281/zenodo.15189495). We thank the COLIBRÍ (FM-GFT) team for alerting us of the afterglow prior to their GCN release.

GCN Circular 44367

Subject
Fermi GRB 260421B: Global MASTER-Net observations report
Date
2026-04-21T05:45:38Z (a month 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-OAGH robotic telescope  [1]  located in Mexico (OAGH National Institute for Astrophysics, Optics and Electronics) started inspect of the Fermi GRB 260421B ( Fermi GBM team, GCN 44360) errorbox  93 sec after notice time and 126 sec after trigger time at 2026-04-21 04:15:27 UT, with upper limit up to  19.9 mag. The observations began at zenith distance = 56 deg. The sun  altitude  is -28.9 deg. 

MASTER-OAFA robotic telescope  located in Argentina (OAFA observatory of San Juan National University) started inspect of the Fermi GRB 260421B errorbox  867 sec after notice time and 900 sec after trigger time at 2026-04-21 04:28:21 UT, with upper limit up to  20.2 mag. The observations began at zenith distance = 21 deg. The sun  altitude  is -70.7 deg. 

The galactic latitude b = 45 deg., longitude l = 298 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=3262990

We obtain a following upper limits.  

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

     142 | 2026-04-21 04:15:27 |         MASTER-OAGH | (12h 53m 10.93s , -17d 20m 23.6s) |   C |    30 | 18.4 |        
     177 | 2026-04-21 04:15:27 |         MASTER-OAGH | (12h 53m 10.93s , -17d 20m 23.5s) |   C |   100 | 19.1 |  Coadd 
     181 | 2026-04-21 04:16:07 |         MASTER-OAGH | (12h 53m 17.82s , -17d 20m 17.3s) |   C |    30 | 18.5 |        
     226 | 2026-04-21 04:16:47 |         MASTER-OAGH | (12h 53m 13.89s , -17d 18m 44.5s) |   C |    40 | 18.4 |        
     282 | 2026-04-21 04:17:37 |         MASTER-OAGH | (12h 53m 13.81s , -17d 20m 10.6s) |   C |    50 | 18.7 |        
     352 | 2026-04-21 04:17:37 |         MASTER-OAGH | (12h 53m 13.81s , -17d 20m 10.6s) |   C |   190 | 19.7 |  Coadd 
     347 | 2026-04-21 04:18:38 |         MASTER-OAGH | (12h 53m 17.74s , -17d 19m 02.7s) |   C |    60 | 18.8 |        
     427 | 2026-04-21 04:19:48 |         MASTER-OAGH | (12h 53m 11.54s , -17d 19m 39.5s) |   C |    80 | 19.2 |        
     538 | 2026-04-21 04:21:28 |         MASTER-OAGH | (12h 35m 06.04s , -17d 47m 57.6s) |   C |   100 | 19.2 |        
     538 | 2026-04-21 04:21:28 |         MASTER-OAGH | (12h 36m 24.29s , -18d 21m 51.5s) |   C |   100 | 19.4 |        
     980 | 2026-04-21 04:28:11 |         MASTER-OAGH | (12h 39m 45.91s , -17d 46m 54.8s) |  P- |   180 | 17.5 |        
     980 | 2026-04-21 04:28:11 |         MASTER-OAGH | (12h 41m 04.22s , -18d 20m 54.9s) |  P| |   180 | 17.7 |        
    1160 | 2026-04-21 04:28:11 |         MASTER-OAGH | (12h 41m 04.22s , -18d 20m 54.8s) |  P| |   540 | 18.8 |  Coadd 
     930 | 2026-04-21 04:28:21 |         MASTER-OAFA | (12h 40m 14.08s , -17d 24m 25.2s) |   C |    60 | 20.0 |        
     996 | 2026-04-21 04:29:26 |         MASTER-OAFA | (12h 40m 14.11s , -17d 24m 23.6s) |   C |    60 | 20.1 |        
    1061 | 2026-04-21 04:30:32 |         MASTER-OAFA | (12h 40m 14.15s , -17d 24m 22.0s) |   C |    60 | 20.1 |        
    1170 | 2026-04-21 04:31:21 |         MASTER-OAGH | (12h 39m 52.24s , -17d 46m 46.8s) |  P- |   180 | 17.7 |        
    1170 | 2026-04-21 04:31:21 |         MASTER-OAGH | (12h 41m 10.67s , -18d 20m 47.4s) |  P| |   180 | 18.1 |        
    1127 | 2026-04-21 04:31:38 |         MASTER-OAFA | (12h 40m 14.20s , -17d 24m 20.9s) |   C |    60 | 20.2 |        
    1193 | 2026-04-21 04:32:44 |         MASTER-OAFA | (12h 40m 14.23s , -17d 24m 19.6s) |   C |    60 | 20.1 |        
    1259 | 2026-04-21 04:33:49 |         MASTER-OAFA | (12h 40m 14.26s , -17d 24m 18.2s) |   C |    60 | 20.1 |        
    1361 | 2026-04-21 04:34:31 |         MASTER-OAGH | (12h 39m 49.24s , -17d 45m 09.3s) |  P- |   180 | 17.7 |        
    1361 | 2026-04-21 04:34:31 |         MASTER-OAGH | (12h 41m 07.78s , -18d 19m 10.2s) |  P| |   180 | 18.3 |        
    1325 | 2026-04-21 04:34:56 |         MASTER-OAFA | (12h 40m 14.30s , -17d 24m 16.6s) |   C |    60 | 20.0 |        
    1391 | 2026-04-21 04:36:01 |         MASTER-OAFA | (12h 40m 14.36s , -17d 24m 15.7s) |   C |    60 | 20.2 |        
    1456 | 2026-04-21 04:37:07 |         MASTER-OAFA | (12h 40m 14.39s , -17d 24m 14.3s) |   C |    60 | 20.1 |        
    1552 | 2026-04-21 04:37:43 |         MASTER-OAGH | (12h 39m 49.10s , -17d 46m 59.8s) |  P- |   180 | 17.9 |        
    1552 | 2026-04-21 04:37:43 |         MASTER-OAGH | (12h 41m 07.75s , -18d 21m 00.3s) |  P| |   180 | 18.3 |        
    1522 | 2026-04-21 04:38:13 |         MASTER-OAFA | (12h 40m 14.44s , -17d 24m 13.3s) |   C |    60 | 20.2 |        
    1588 | 2026-04-21 04:39:18 |         MASTER-OAFA | (12h 40m 14.44s , -17d 24m 12.1s) |   C |    60 | 20.1 |        
    1653 | 2026-04-21 04:40:24 |         MASTER-OAFA | (12h 40m 14.48s , -17d 24m 10.8s) |   C |    60 | 20.1 |        
    1742 | 2026-04-21 04:40:53 |         MASTER-OAGH | (12h 39m 51.19s , -17d 45m 34.9s) |  P- |   180 | 17.7 |        
    1742 | 2026-04-21 04:40:53 |         MASTER-OAGH | (12h 41m 09.87s , -18d 19m 35.1s) |  P| |   180 | 18.4 |        
    1720 | 2026-04-21 04:41:30 |         MASTER-OAFA | (12h 40m 14.54s , -17d 24m 09.9s) |   C |    60 | 20.1 |        
    1785 | 2026-04-21 04:42:36 |         MASTER-OAFA | (12h 40m 14.54s , -17d 24m 08.9s) |   C |    60 | 20.1 |        
    1850 | 2026-04-21 04:43:41 |         MASTER-OAFA | (12h 40m 14.57s , -17d 24m 07.9s) |   C |    60 | 20.0 |        
    1874 | 2026-04-21 04:44:05 |         MASTER-OAGH | (12h 34m 22.11s , -17d 48m 09.2s) |   C |    60 | 19.5 |        
    1874 | 2026-04-21 04:44:05 |         MASTER-OAGH | (12h 35m 41.12s , -18d 22m 06.8s) |   C |    60 | 19.7 |        
    1917 | 2026-04-21 04:44:48 |         MASTER-OAFA | (12h 40m 14.60s , -17d 24m 07.3s) |   C |    60 | 20.1 |        
    1946 | 2026-04-21 04:45:17 |         MASTER-OAGH | (12h 25m 58.60s , -17d 47m 08.9s) |   C |    60 | 19.3 |        
    1946 | 2026-04-21 04:45:17 |         MASTER-OAGH | (12h 27m 17.78s , -18d 21m 05.4s) |   C |    60 | 19.7 |        
    1982 | 2026-04-21 04:45:53 |         MASTER-OAFA | (12h 40m 14.65s , -17d 24m 05.9s) |   C |    60 | 20.1 |        
    2047 | 2026-04-21 04:46:58 |         MASTER-OAFA | (12h 40m 14.66s , -17d 24m 04.9s) |   C |    60 | 20.1 |        
    2113 | 2026-04-21 04:48:04 |         MASTER-OAFA | (12h 40m 14.68s , -17d 24m 03.9s) |   C |    60 | 20.1 |        
    2179 | 2026-04-21 04:49:09 |         MASTER-OAFA | (12h 40m 14.72s , -17d 24m 02.9s) |   C |    60 | 20.2 |        
    2245 | 2026-04-21 04:50:15 |         MASTER-OAFA | (12h 40m 14.76s , -17d 24m 02.1s) |   C |    60 | 20.2 |        
    2311 | 2026-04-21 04:51:22 |         MASTER-OAFA | (12h 40m 14.77s , -17d 24m 00.8s) |   C |    60 | 20.1 |        
    2376 | 2026-04-21 04:52:27 |         MASTER-OAFA | (12h 40m 14.82s , -17d 24m 00.2s) |   C |    60 | 20.2 |        
    2442 | 2026-04-21 04:53:32 |         MASTER-OAFA | (12h 40m 14.86s , -17d 23m 59.2s) |   C |    60 | 20.2 |        
    2474 | 2026-04-21 04:54:05 |         MASTER-OAGH | (12h 34m 21.59s , -17d 48m 11.5s) |   C |    60 | 19.4 |        
    2474 | 2026-04-21 04:54:05 |         MASTER-OAGH | (12h 35m 40.80s , -18d 22m 07.1s) |   C |    60 | 19.8 |        
    2508 | 2026-04-21 04:54:39 |         MASTER-OAFA | (12h 40m 14.86s , -17d 23m 58.4s) |   C |    60 | 20.1 |        
    2546 | 2026-04-21 04:55:17 |         MASTER-OAGH | (12h 25m 58.36s , -17d 49m 11.4s) |   C |    60 | 19.5 |        
    2546 | 2026-04-21 04:55:17 |         MASTER-OAGH | (12h 27m 17.73s , -18d 23m 06.0s) |   C |    60 | 19.7 |        
    2574 | 2026-04-21 04:55:44 |         MASTER-OAFA | (12h 40m 14.88s , -17d 23m 57.6s) |   C |    60 | 20.0 |        
    2640 | 2026-04-21 04:56:50 |         MASTER-OAFA | (12h 40m 14.94s , -17d 23m 56.9s) |   C |    60 | 20.1 |        
    2706 | 2026-04-21 04:57:57 |         MASTER-OAFA | (12h 40m 14.96s , -17d 23m 55.9s) |   C |    60 | 20.0 |        
    2771 | 2026-04-21 04:59:02 |         MASTER-OAFA | (12h 40m 15.00s , -17d 23m 54.7s) |   C |    60 | 20.0 |        
    2837 | 2026-04-21 05:00:08 |         MASTER-OAFA | (12h 40m 15.01s , -17d 23m 53.9s) |   C |    60 | 19.9 |        
    2903 | 2026-04-21 05:01:14 |         MASTER-OAGH | (12h 34m 23.48s , -19d 42m 11.3s) |   C |    60 | 18.8 |        
    2903 | 2026-04-21 05:01:14 |         MASTER-OAGH | (12h 35m 43.55s , -20d 16m 06.9s) |   C |    60 | 19.0 |        
    2903 | 2026-04-21 05:01:14 |         MASTER-OAFA | (12h 40m 15.04s , -17d 23m 53.2s) |   C |    60 | 20.0 |        
    2968 | 2026-04-21 05:02:19 |         MASTER-OAFA | (12h 40m 15.06s , -17d 23m 52.6s) |   C |    60 | 20.0 |        
    2974 | 2026-04-21 05:02:25 |         MASTER-OAGH | (12h 42m 52.05s , -19d 41m 12.6s) |   C |    60 | 18.6 |        
    2974 | 2026-04-21 05:02:25 |         MASTER-OAGH | (12h 44m 12.00s , -20d 15m 09.4s) |   C |    60 | 19.1 |        
    3034 | 2026-04-21 05:03:25 |         MASTER-OAFA | (12h 40m 15.10s , -17d 23m 51.8s) |   C |    60 | 20.0 |        
    3100 | 2026-04-21 05:04:31 |         MASTER-OAFA | (12h 40m 15.14s , -17d 23m 51.1s) |   C |    60 | 19.9 |        
    3106 | 2026-04-21 05:04:37 |         MASTER-OAGH | (12h 34m 20.33s , -17d 47m 13.7s) |   C |    60 | 19.1 |        
    3106 | 2026-04-21 05:04:37 |         MASTER-OAGH | (12h 35m 39.73s , -18d 21m 08.6s) |   C |    60 | 19.2 |        
    3166 | 2026-04-21 05:05:37 |         MASTER-OAFA | (12h 40m 15.18s , -17d 23m 50.7s) |   C |    60 | 20.0 |        
    3177 | 2026-04-21 05:05:47 |         MASTER-OAGH | (12h 26m 02.37s , -17d 48m 11.9s) |   C |    60 | 19.3 |        
    3177 | 2026-04-21 05:05:47 |         MASTER-OAGH | (12h 27m 21.94s , -18d 22m 05.5s) |   C |    60 | 19.6 |        
    3232 | 2026-04-21 05:06:42 |         MASTER-OAFA | (12h 40m 15.19s , -17d 23m 50.1s) |   C |    60 | 20.0 |        
    3298 | 2026-04-21 05:07:49 |         MASTER-OAFA | (12h 40m 15.22s , -17d 23m 49.4s) |   C |    60 | 20.1 |        
    3307 | 2026-04-21 05:07:58 |         MASTER-OAGH | (12h 34m 24.99s , -15d 54m 14.3s) |   C |    60 | 19.5 |        
    3307 | 2026-04-21 05:07:58 |         MASTER-OAGH | (12h 35m 43.97s , -16d 28m 07.6s) |   C |    60 | 19.6 |        
    3363 | 2026-04-21 05:08:54 |         MASTER-OAFA | (12h 40m 15.24s , -17d 23m 48.3s) |   C |    60 | 20.0 |        
    3378 | 2026-04-21 05:09:08 |         MASTER-OAGH | (12h 42m 36.90s , -15d 55m 16.1s) |   C |    60 | 19.3 |        
    3378 | 2026-04-21 05:09:08 |         MASTER-OAGH | (12h 43m 55.82s , -16d 29m 09.9s) |   C |    60 | 19.6 |        
    3429 | 2026-04-21 05:09:59 |         MASTER-OAFA | (12h 40m 15.26s , -17d 23m 47.3s) |   C |    60 | 20.0 |        
    3495 | 2026-04-21 05:11:05 |         MASTER-OAFA | (12h 40m 15.29s , -17d 23m 46.6s) |   C |    60 | 20.0 |        
    3509 | 2026-04-21 05:11:20 |         MASTER-OAGH | (12h 34m 22.74s , -19d 41m 12.5s) |   C |    60 | 19.6 |        
    3509 | 2026-04-21 05:11:20 |         MASTER-OAGH | (12h 35m 43.08s , -20d 15m 06.9s) |   C |    60 | 19.9 |        
    3560 | 2026-04-21 05:12:11 |         MASTER-OAFA | (12h 40m 15.29s , -17d 23m 46.2s) |   C |    60 | 20.0 |        
    3581 | 2026-04-21 05:12:32 |         MASTER-OAGH | (12h 42m 57.79s , -19d 42m 14.3s) |   C |    60 | 19.6 |        
    3581 | 2026-04-21 05:12:32 |         MASTER-OAGH | (12h 44m 18.06s , -20d 16m 09.5s) |   C |    60 | 19.7 |        
    3626 | 2026-04-21 05:13:17 |         MASTER-OAFA | (12h 40m 15.31s , -17d 23m 45.6s) |   C |    60 | 20.0 |        
    3692 | 2026-04-21 05:14:23 |         MASTER-OAFA | (12h 40m 15.34s , -17d 23m 45.0s) |   C |    60 | 20.0 |        
    3719 | 2026-04-21 05:14:50 |         MASTER-OAGH | (12h 34m 21.06s , -17d 47m 14.4s) |   C |    60 | 19.6 |        
    3719 | 2026-04-21 05:14:50 |         MASTER-OAGH | (12h 35m 40.72s , -18d 21m 07.3s) |   C |    60 | 19.9 |        
    3758 | 2026-04-21 05:15:29 |         MASTER-OAFA | (12h 40m 15.36s , -17d 23m 44.7s) |   C |    60 | 20.1 |        
    3791 | 2026-04-21 05:16:02 |         MASTER-OAGH | (12h 26m 01.87s , -17d 48m 12.2s) |   C |    60 | 19.3 |        
    3791 | 2026-04-21 05:16:02 |         MASTER-OAGH | (12h 27m 21.65s , -18d 22m 03.8s) |   C |    60 | 19.3 |        
    3823 | 2026-04-21 05:16:34 |         MASTER-OAFA | (12h 40m 15.39s , -17d 23m 44.0s) |   C |    60 | 20.1 |        
    3889 | 2026-04-21 05:17:40 |         MASTER-OAFA | (12h 40m 15.43s , -17d 23m 43.4s) |   C |    60 | 20.1 |        
    3930 | 2026-04-21 05:18:21 |         MASTER-OAGH | (12h 34m 17.86s , -15d 53m 14.7s) |   C |    60 | 19.4 |        
    3930 | 2026-04-21 05:18:21 |         MASTER-OAGH | (12h 35m 37.05s , -16d 27m 06.0s) |   C |    60 | 19.5 |        
    3955 | 2026-04-21 05:18:45 |         MASTER-OAFA | (12h 40m 15.45s , -17d 23m 42.8s) |   C |    60 | 20.2 |        
    4002 | 2026-04-21 05:19:32 |         MASTER-OAGH | (12h 42m 42.14s , -15d 54m 17.4s) |   C |    60 | 18.6 |        
    4002 | 2026-04-21 05:19:32 |         MASTER-OAGH | (12h 44m 01.28s , -16d 28m 08.9s) |   C |    60 | 18.9 |        
    4021 | 2026-04-21 05:19:51 |         MASTER-OAFA | (12h 40m 15.46s , -17d 23m 42.1s) |   C |    60 | 20.1 |        
    4087 | 2026-04-21 05:20:58 |         MASTER-OAFA | (12h 40m 15.48s , -17d 23m 41.6s) |   C |    60 | 20.1 |        
    4141 | 2026-04-21 05:21:51 |         MASTER-OAGH | (12h 34m 22.53s , -19d 41m 12.5s) |   C |    60 | 16.1 |        
    4141 | 2026-04-21 05:21:51 |         MASTER-OAGH | (12h 35m 43.04s , -20d 15m 04.4s) |   C |    60 | 15.7 |        
    4153 | 2026-04-21 05:22:04 |         MASTER-OAFA | (12h 40m 15.50s , -17d 23m 41.3s) |   C |    60 | 20.0 |        
    4212 | 2026-04-21 05:23:03 |         MASTER-OAGH | (12h 42m 58.08s , -19d 42m 15.1s) |   C |    60 | 18.5 |        
    4212 | 2026-04-21 05:23:03 |         MASTER-OAGH | (12h 44m 18.55s , -20d 16m 07.7s) |   C |    60 | 18.1 |        
    4219 | 2026-04-21 05:23:09 |         MASTER-OAFA | (12h 40m 15.52s , -17d 23m 40.6s) |   C |    60 | 20.1 |        
    4285 | 2026-04-21 05:24:15 |         MASTER-OAFA | (12h 40m 15.55s , -17d 23m 40.1s) |   C |    60 | 20.0 |        
    4341 | 2026-04-21 05:25:12 |         MASTER-OAGH | (12h 34m 18.76s , -17d 47m 13.7s) |   C |    60 | 18.2 |        
    4341 | 2026-04-21 05:25:12 |         MASTER-OAGH | (12h 35m 38.65s , -18d 21m 04.7s) |   C |    60 | 17.9 |        
    4350 | 2026-04-21 05:25:20 |         MASTER-OAFA | (12h 40m 15.59s , -17d 23m 39.6s) |   C |    60 | 20.0 |        
    4412 | 2026-04-21 05:26:23 |         MASTER-OAGH | (12h 26m 01.58s , -17d 48m 10.3s) |   C |    60 | 15.8 |        
    4412 | 2026-04-21 05:26:23 |         MASTER-OAGH | (12h 27m 21.61s , -18d 22m 00.1s) |   C |    60 | 16.0 |        
    4416 | 2026-04-21 05:26:26 |         MASTER-OAFA | (12h 40m 15.62s , -17d 23m 39.2s) |   C |    60 | 20.1 |        
    4482 | 2026-04-21 05:27:33 |         MASTER-OAFA | (12h 40m 15.64s , -17d 23m 38.8s) |   C |    60 | 19.9 |        
    4542 | 2026-04-21 05:28:33 |         MASTER-OAGH | (12h 34m 16.69s , -15d 53m 12.9s) |   C |    60 | 14.3 |        
    4547 | 2026-04-21 05:28:38 |         MASTER-OAFA | (12h 40m 15.67s , -17d 23m 38.2s) |   C |    60 | 20.0 |        
    4613 | 2026-04-21 05:29:44 |         MASTER-OAGH | (12h 42m 41.80s , -15d 54m 16.3s) |   C |    60 | 15.3 |        
    4613 | 2026-04-21 05:29:44 |         MASTER-OAGH | (12h 44m 01.27s , -16d 28m 06.0s) |   C |    60 | 14.9 |        
    4613 | 2026-04-21 05:29:44 |         MASTER-OAFA | (12h 40m 15.70s , -17d 23m 37.8s) |   C |    60 | 19.9 |        
    4679 | 2026-04-21 05:30:50 |         MASTER-OAFA | (12h 40m 15.74s , -17d 23m 37.2s) |   C |    60 | 20.0 |        
    4743 | 2026-04-21 05:31:54 |         MASTER-OAGH | (12h 34m 21.40s , -19d 41m 10.3s) |   C |    60 | 14.6 |        
    4743 | 2026-04-21 05:31:54 |         MASTER-OAGH | (12h 35m 42.22s , -20d 15m 00.7s) |   C |    60 | 14.5 |        
    4745 | 2026-04-21 05:31:55 |         MASTER-OAFA | (12h 40m 15.77s , -17d 23m 37.0s) |   C |    60 | 20.0 |        
    4810 | 2026-04-21 05:33:01 |         MASTER-OAFA | (12h 40m 15.79s , -17d 23m 36.5s) |   C |    60 | 20.1 |        
    4815 | 2026-04-21 05:33:05 |         MASTER-OAGH | (12h 42m 56.35s , -19d 42m 13.4s) |   C |    60 | 15.5 |        
    4815 | 2026-04-21 05:33:05 |         MASTER-OAGH | (12h 44m 17.15s , -20d 16m 04.5s) |   C |    60 | 15.2 |        
    4876 | 2026-04-21 05:34:07 |         MASTER-OAFA | (12h 40m 15.81s , -17d 23m 36.3s) |   C |    60 | 20.0 |        
    4941 | 2026-04-21 05:35:12 |         MASTER-OAFA | (12h 40m 15.84s , -17d 23m 36.4s) |   C |    60 | 20.0 |        
    4954 | 2026-04-21 05:35:24 |         MASTER-OAGH | (12h 34m 19.16s , -17d 47m 10.7s) |   C |    60 | 14.1 |        
    5007 | 2026-04-21 05:36:17 |         MASTER-OAFA | (12h 40m 15.86s , -17d 23m 35.7s) |   C |    60 | 20.0 |        
    5073 | 2026-04-21 05:37:24 |         MASTER-OAFA | (12h 40m 15.87s , -17d 23m 35.3s) |   C |    60 | 19.9 |        
    5138 | 2026-04-21 05:38:29 |         MASTER-OAFA | (12h 40m 15.91s , -17d 23m 35.2s) |   C |    60 | 19.9 |        
    5184 | 2026-04-21 05:39:15 |         MASTER-OAGH | (12h 34m 16.09s , -15d 53m 00.5s) |   C |    60 | 17.1 |        
    5184 | 2026-04-21 05:39:15 |         MASTER-OAGH | (12h 35m 37.14s , -16d 26m 52.1s) |   C |    60 | 17.1 |        
    5204 | 2026-04-21 05:39:35 |         MASTER-OAFA | (12h 40m 15.93s , -17d 23m 34.8s) |   C |    60 | 19.9 |        
    5256 | 2026-04-21 05:40:27 |         MASTER-OAGH | (12h 42m 39.48s , -15d 54m 07.1s) |   C |    60 | 17.1 |        
    5256 | 2026-04-21 05:40:27 |         MASTER-OAGH | (12h 44m 00.43s , -16d 27m 59.1s) |   C |    60 | 16.8 |        
    5270 | 2026-04-21 05:40:41 |         MASTER-OAFA | (12h 40m 15.94s , -17d 23m 34.3s) |   C |    60 | 19.9 |        
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 44366

Subject
GRB 260421B: BOOTES-7 early optical afterglow detection
Date
2026-04-21T05:37:53Z (a month ago)
From
Alberto Castro-Tirado at Inst.de Astro. de Andalucia <ajct@iaa.es>
Via
email
S.-Y. Wu, M. Gritsevich, I. Perez-Garcia, E.  Fernandez-Garcia, M. D. Caballero-Garcia, R. Sanchez-Ramirez, S. Guziy, and A. J. Castro-Tirado (IAA-CSIC,  Granada), C. Perez del Pulgar (Univ. of Malaga), Y.-D. Hu (GXU, Guanxi), B.-B. Zhang (Nanjing Univ.), and A. Maury (Space Obs., San Pedro de Atacama), on behalf of a larger  collaboration, report:

Following the detection of GRB 260421B by Fermi (Fermi Team, GCN 44360, Preis and Greiner, GCN 44361) and SVOM (Zhao et al. GCN 44362), the BOOTES-7 telescope at  the 0.6m BOOTES-7 robotic telescope at Space Observatory (San Pedro de Atacama, Chile) automatically observed the GRB location starting on Aug. 21, 04:28:36 UT (i.e. 15 min after GRB onset).

A series of images in clear filter were gathered and we identify the fading optical afterglow reported by He et al. (GCN 44363) Sánchez-Álvarez et al. (GCN 44364) and Turpin et al. (GCN 44365) at coordinates RA: 12:40:00.50 Dec: -17:44:24.0 (+/- 0.5 “). Preliminary magnitude is 18. Further data analysis is ongoing.

We would like to thank the staff at San Pedro de Atacama Space Observatory for their excellent support.

GCN Circular 44365

Subject
GRB 260421B: LCO optical afterglow confirmation
Date
2026-04-21T05:32:17Z (a month ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
D. Turpin (CEA/Irfu), D. B. Malesani (DAWN/NBI and Radboud), A. Saccardi (CEA/Irfu), C. Wu (NAOC), J. T. Palmerio (CEA/Irfu), L. P. Xin (NAOC), B. Cordier (CEA/Irfu), W.-L Zhang (PMO), Z.-Y Liu (GZNU), report on behalf of the SVOM mission team:

We observed the field of GRB 260421B detected by SVOM (Zhao et al., GCN 44362) and Fermi/GBM (The Fermi GBM team, GCN 44360) with the LCO 1m telescope at Cerro Tololo Observatory equipped with the Sinistro instrument.

Our observation started at 2026-04-21T04:29:54.940 about 16.5 min after the SVOM/ECLAIRs trigger time. We obtained 3x200 s exposures in the SDSS r and PanSTARRS-z filters. An uncatalogued bright optical candidate is well detected in our single images at RA, dec (J2000) = 190.00203, -17.74017 (err = 0.5 arcsec). This position is consistent with the optical counterpart reported by the TRT telescope (He et al, GCN 44363) and the SVOM/COLIBRI FM-GFT (Álvarez et al. GCN 43646).

We measure the following magnitude calibrated against the PanSTARRS DR1 catalog, and not corrected for Galactic extinction:

r = 18.47 +/- 0.01 (AB) (Tmid - T0 = 18.2 min)
z = 18.92 +/- 0.09 (AB) (Tmid - T0 = 30.7 min)

This project is funded by the SVOM collaboration. Further observations are scheduled.

GCN Circular 44364

Subject
GRB 260421B: SVOM/COLIBRÍ (FM-GFT) optical detection of a candidate counterpart
Date
2026-04-21T05:28:06Z (a month ago)
From
freddalvarez@astro.unam.mx
Via
Web form
Fredd Sánchez Álvarez (UNAM), Nikos Mandarakas (LAM), Alan M. Watson (UNAM), Noémie Globus (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), Rosa L. Becerra (UNAM), 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), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), W.L. Zhang  (PMO), and Z. Y. Liu (GZNU) report:

We imaged the field of the SVOM/ECLAIRs and Fermin/GBM GRB 260421B (Zhao et al., GCN Circ. 44362; Fermi GBM Team, GCN Circ. 44360) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-04-21 04:15:13 to 04:30:07 (from 1.9 to 16.8 minutes after the trigger) and obtained 430 seconds of simultaneous exposure in the r/z filters.

The data were reduced, coadded, calibrated, and analyzed with the COLIBRÍ 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.

We detect an uncatalogued source consistent with the ECLAIRs localization at: 
RA(J2000) = 12:40:00.48 = 190.00200 degrees
Dec(J2000) = -17:44:24.6 = -17.74015 degrees
with an uncertainty of 0.5 arcsec.

The source was detected in both r and z and was observed to fade from r ≈ 14.5 to r ≈ 18.3 during our observations. The color of the source was r - z ≈ 0.45 +/- 0.02, which is consistent with the expectations for an afterglow. We suggest this is the optical counterpart of the GRB.

Subsequent imaging in the g/r/i/z/y filters show detections in all bands, indicating that the redshift is less than 3.5.

We encourage spectroscopic observations of the source. Further observations are planned.
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 44363

Subject
GRB 260421B: TRT optical counterpart detection
Date
2026-04-21T05:26:32Z (a month ago)
From
L. B. He at NAOC <helb@bao.ac.cn>
Via
Web form
L.B. He (NAOC), K. Noysena, K. Chanchaiworawit, S. Tinyanont (NARIT), S.Y. Fu (HUST), X. Liu, Z.P. Zhu, S.Q. Jiang, J. An, D. Xu (NAOC) report:

We observed the field of GRB 260421B detected by Fermi (GCN 44360) and SVOM (zhao et al., GCN 44362), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Cerro Tololo Inter-American Observatory, Chile (CTO).  We obtained a series of frames in the r-band. 

An uncatalogued source is detected within the Fermi error circle at coordinates
R.A. (J2000) = 12:40:00.5
Dec. (J2000) = -17:44:24.5
with an uncertainty of ~ 1.0 arcsec. The source had r = 19.14 +/- 0.07  mag, calibrated with Pan-STARRS DR2 and not corrected for Galactic extinction.

No NEO would be at the above position at the observational time by checking MPC.

We thus think the source is the optical counterpart of GRB 260421B.

GCN Circular 44362

Subject
GRB 260421B: SVOM detection of a long burst
Date
2026-04-21T05:20:23Z (a month ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
D.Zhao (NAOC), H.Goto (RIKEN),  W. L. Zhang (PMO),  Z. Y. Liu (GZNU) report on behalf of the SVOM mission team:


At 2026-04-21T04:13:25 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 260421B (SVOM burst-id sb26042105). The Fermi GBM also detected this burst (GCN 44360 & 44361).

The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network.

The burst was detected both by the Count-Rate Trigger (CRT) and the Image Trigger (IMT), which produced a sequence of 13 alerts. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 30.71 in the [8-120] keV energy band over a time window of 20.48 seconds starting at 2026-04-21T04:13:17. 
The localization of the best alert is R.A., Dec. 189.9811, -17.7676 degrees (J2000) with a 90% confidence level (C.L.) radius of 3.21 arcmin (including systematic error of 2 arcmin added in quadrature). 

The SVOM/ECLAIRs light curve showed a multiple peak structure with a T90 duration of about 52.824 (-0.172 +0.169).

This burst also triggered SVOM/GRM at 2026-04-21T04:13:21 on a timescale of 1 seconds with an SNR of 7.90.
The SVOM/GRM light curve showed a multiple peak structure with a T90 duration of about 20.04 (-0.93 +3.07).

SVOM slewed to the burst.

SVOM/MXT began observing the field at 2026-04-21T04:18:40 UTC, 315 seconds after T0. Using onboard processed data we found an uncatalogued X-ray source located at R.A., Dec. 190.0318, -17.7566 degrees:
R.A. (J2000) = 12h40m07.62s
Dec. (J2000) = -17d45m23.69s
with a 90% C.L. radius of 97.27 arcseconds.

This location is 2.97 arcminutes from the ECLAIRs onboard position. This position may be improved as more data is received.

VT began observing the field after the slew. The analysis of the data will be published in a future circular.

The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. SVOM/ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. SVOM/GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. SVOM/MXT was developed jointly by CNES, CEA-IRFU, CNRS-IJCLab, University of Leicester, MPE.

The Burst Advocate (BA) on shift for this alert is W. L. Zhang: wlzhang@pmo.ac.cn
Please contact the BA by email if you require additional information.

GCN Circular 44361

Subject
GRB 260421B: BALROG localization (Fermi Trigger 798437606 / GRB 260421176)
Date
2026-04-21T04:54:24Z (a month ago)
From
Jochen Greiner at MPE <jcgrog@mpe.mpg.de>
Via
email
T. Preis (University of Innsbruck) & J. Greiner (MPE Garching) report:

The public trigdat data of the Fermi Gamma-ray Burst Monitor (GBM) trigger
798437606 at 04:13:21 on 21 April 2026 were automatically fitted for spectrum
and sky location with BALROG (Burgess et al. 2018, MNRAS 476, 1427;
Berlato et al. 2019, ApJ 873, 60).

The best-fit position is:
RA(2000.0) = 190.5 deg
Decl.(2000.0) = -16.5 deg
The 1 sigma statistical error radius is 2.2 deg.
We estimate an additional systematic error of 1 deg.

Further details are available at:
https://grb.mpe.mpg.de/grb/GRB260421176/

The Healpix map can be downloaded from:
https://grb.mpe.mpg.de/grb/GRB260421176/healpix

The location parameters are available as JSON at:
https://grb.mpe.mpg.de/grb/GRB260421176/json

GCN Circular 44360

Subject
GRB 260421B: Fermi GBM Final Real-time Localization
Date
2026-04-21T04:22:36Z (a month 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 04:13:21 UT on 21 Apr 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260421B (trigger 798437606.166588 / 260421176).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 188.8, Dec = -17.8 (J2000 degrees, equivalent to J2000 12h 35m, -17d 48'), with a statistical uncertainty of 2.4 degrees.

The angle from the Fermi LAT boresight is 58.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260421176/quicklook/glg_skymap_all_bn260421176.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/bn260421176/quicklook/glg_healpix_all_bn260421176.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260421176/quicklook/glg_lc_medres34_bn260421176.gif
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