GRB 260603A
GCN Circular 44848
Subject
GRB260603A: SVOM/ECLAIRs refined analysis
Date
2026-06-06T09:24:50Z (5 months ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
M. Brunet (IRAP),N. Dagoneau (CEA), Donghua ZHAO, Wenjin Xie (NAOC), Ruizhi Li (YNAO), Ziming Wang (BNU) 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 260603A (SVOM burst-id sb26060303 – GCN 44787, trigger time T0 = 2026-06-03T05:38:16 UTC).
The burst that triggered ECLAIRs onboard shows a single peak lightcurve. The burst duration is T90 = 101 +4/-7 s in the 4-120 keV energy band.
The time-averaged spectrum from T0-51.0 s to T0+50.7 s in the 5-120 keV energy range is best fitted by a powerlaw model with alpha = -1.75 +0.08/-0.09. With this model, the 4-120 keV fluence is (1.33 +0.11/-0.12)e-6 erg/cm^2 and the 4-120 keV photon flux is 0.47 +0.02/-0.03 ph/cm^2/s.
All the quoted errors are at the 68% confidence level.
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 44820
Subject
GRB 260603A: NOT optical observations
Date
2026-06-04T12:05:46Z (5 months ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
D. B. Malesani (DAWN/NBI and Radboud), L. Izzo (INAF/OACN and DARK/NBI), G. Corcoran (UCD), A. Martin-Carrillo (UCD), N. Pyykkinen (NOT), A. S. Mårtensson (NOT) report on behalf of a larger collaboration:
We observed the optical afterglow (Guelfand et al., GCNs 44791, 44797; Li et al., GCN 44799) of the SVOM GRB 260603A (Zhao et al., GCN 44787) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations were carried out in the SDSS r band.
The optical afterglow is detected with an AB magnitude r = 23.27 +- 0.21, at a mean epoch of Jun 4.02 UT (0.786 days after trigger). This magnitude is calibrated against nearby stars from the Pan-STARRS catalog, and is not corrected for Galactic extinction.
GCN Circular 44819
Subject
GRB 260603A: Simeiz Zeiss-1000 optical upper limit
Date
2026-06-04T09:46:23Z (5 months ago)
From
Nicolai Pankov at IKI <colinsergesen@gmail.com>
Via
Web form
N. Pankov (IKI), A. Pozanenko (IKI), A. Novichonok (KIAM), A. Volnova (IKI) report on behalf of IKI-GRB-FuN
We observed the field of GRB 260603A (Zhao et al., GCN 44787; Li et al., GCN 44805) with the Zeiss-1000 1-meter telescope of Simeiz Observatory starting on 2026-06-03 UT 19:38:36 (~0.6 days since trigger). A series of 20x180 sec exposures was acquired in the R-filter using a CCD-photometer. We do not detect the optical afterglow (Guelfand et al., GCN 44791, 44797; Li et al., GCN 44799; Freeberg et al., GCN 44800). The preliminary upper limit and the observation details are summarized below:
Date UTstart t-T0 Exp. Filter Mag Err. UL
(mid,days) (n*s) (3-sigma)
2026-06-03 19:38:36 0.60443 20*180 R n/d n/d 22.3
The photometry is based on nearby USNO-B1.0 stars and is not corrected for the Galactic extinction.
GCN Circular 44805
Subject
GRB 260603A: EP-FXT follow-up observation
Date
2026-06-03T14:21:26Z (5 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Z. X. Li, G. L. Huang, J. Y. Cao (IHEP, CAS), J. W. Hu, Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team:
EP-FXT performed an automatic follow-up observation of GRB 260603A detected by SVOM/ECLAIRs (SVOM/sb26051504, ZHAO et al., GCN 44787). The follow-up observation started at 2026-06-03T07:38:06 UTC, approximately 2 hours after the SVOM trigger, with a total exposure time of 2.4 ks.
On-ground analysis of the FXT data found an uncatalogued source within the ECLAIRs error circle at R.A., Dec. = 233.1435, 40.0258 deg (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). This position is consistent with the optical counterpart (Guelfand et al., GCN 44791). The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power-law model, with the hydrogen column density of 1.7 (-0.8/+0.8) × 10^21 cm^-2 and a photon index of 2.4 (-0.3/+0.3). The derived average unabsorbed flux in the 0.5-10 keV band is 4.0 (-0.4, +0.5) x 10^(-12) erg/s/cm^2.
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 44800
Subject
GRB 260603A: Kilonova-Catcher optical upper limit
Date
2026-06-03T12:36:36Z (5 months ago)
From
Damien Turpin at CEA-Saclay <dturpin-astro@hotmail.com>
Via
Web form
M. Freeberg (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), S. Antier (IJCLab), M. Coughlin (UMN), S. Karpov (FZU), P. Hello (IJCLAB), M. Pillas (IAP) on behalf of the GRANDMA/Kilonova-Catcher collaboration:
We observed the field of GRB 260603A detected by SVOM/ECLAIRs (Zhao et al., GCN 44787) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with the T11 itelescope located at the Utah Desert Remote Observatory (USA) and operated by M. Freeberg. Our observations started at TGRB+1.78 hours and were taken with the sdssr filter.
In our stacked frames, subtracted from the PanSTARRS DR2 template image, we do not detect the optical counterpart reported by SVOM/COLIBRÍ (FM-GFT) (Guelfand et al., GCN 44791; GCN 44797) and SVOM/VT (Li et al., GCN 44799).
We report our follow-up results in the table below:
| Tmid-TGRB (hr) | Exp (s) | Filter | Magnitude | Instrument |
|---|---|---|---|---|
| 2.25 | 11 x 300s | r (AB) | 20.1 (3 sigma) | iT11 |
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 44799
Subject
GRB 260603A: SVOM/VT optical observations
Date
2026-06-03T12:00:59Z (5 months ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H. L. Li, C. Wu, Y. L. Qiu, J. R. Xu, L. P. Xin, Y. N. Ma, Z. H. Yao, 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), R. Z. Li (YNAO) and Z. M. Wang (BNU) report on behalf of the SVOM/VT team.
SVOM/VT performed ToO observations to the field of GRB 260603A triggered by SVOM/ECLAIRs (sb26060303, Zhao et al., GCN 44787). The observation started at 2026-06-03T07:04:07 UTC, 1.44 hours post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.
The optical counterpart (Guelfand et al., GCN 44791; GCN 44797) was detected by VT in both channels. The following measurements are in the AB magnitude without correction for Galactic extinction:
Mid time | Band | Exposure Time | Brightness
1.78 h VT_B 36*70 s 22.08 +/- 0.12 mag
1.78 h VT_R 30*70 s 21.37 +/- 0.10 mag
3.41 h VT_B 36*70 s 22.40 +/- 0.14 mag
3.40 h VT_R 31*70 s 21.74 +/- 0.11 mag
The source showed a fading slope of about -0.53 during the observations above.
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 44797
Subject
GRB 260603A: SVOM/COLIBRÍ (FM-GFT) photometric redshift z = 3.57
Date
2026-06-03T10:00:53Z (5 months ago)
From
Marion G <marion.guelfand@gmail.com>
Via
Web form
Marion Guelfand (CPPM), Alan M. Watson (UNAM), Jean-Grégoire Ducoin (CPPM), Benjamin Schneider (LAM), 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), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Antonio de Ugarte Postigo (LAM), Ruizhi Li (YNAO), and Ziming Wang (BNU) report:
We continued to image the field of the SVOM/ECLAIRs GRB 260603A (Zhao et al., GCN Circ. 44787) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-06-03 06:27 to 07:24 UTC (from 0.83 to 1.79 hours after the trigger) and obtained 16, 16, 16, 16, and 32 minutes of exposure in the g, r, i, z, and y filters, respectively.
The data were reduced and coadded 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.
The candidate counterpart reported by Guelfand et al. (GCN Circ. 44791) is detected with preliminary magnitudes of:
g = 21.97 +/- 0.10
r = 21.23 +/- 0.05
i = 20.96 +/- 0.05
z = 20.87 +/- 0.09
y = 20.91 +/- 0.18
The counterpart has continued to fade.
After correcting for the Galactic foreground extinction corresponding to a reddening of E(B-V) = 0.019 mag (Schlafly & Finkbeiner 2011) and fitting a power-law model to the grizy-bands using no dust, SMC, LMC and MW extinction curves, we derive a photometric redshift of z = 3.57 +- 0.42 (1-sigma c.l.).
We note the presence of a galaxy about 3.5 arcsec to the north of the counterpart. This galaxy is seen in both Pan-STARRS DR2 and LS DR10 images and has g ≈ 22.11 +/- 0.04, r ≈ 21.33 +/- 0.03, and z ≈ 20.93 +/- 0.04 in the LS DR10. However, the photometric redshift in the LS DR9 is 0.343 +/- 0.088, which is not compatible with the derived photometric redshift of the counterpart, and so this galaxy is unlikely to be the host.
On the other hand, a deep g-band image from the Hyper Suprime-Cam Legacy Archive (HSCLA, Tanaka et al. 2021) reveals an underlying object at a position consistent with the optical counterpart, suggesting a possible host galaxy.
We encourage spectroscopic observations of the counterpart.
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 44791
Subject
GRB 260603A: SVOM/COLIBRÍ (GFT-FM) optical counterpart candidate
Date
2026-06-03T06:54:17Z (5 months ago)
Edited On
2026-06-03T13:33:20Z (5 months ago)
From
Marion G <marion.guelfand@gmail.com>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Marion G <marion.guelfand@gmail.com>
Via
Web form
Marion Guelfand (CPPM), 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), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), Ruizhi Li (YNAO), and Ziming Wang (BNU) report:
We imaged the field of the SVOM GRB 260603A (Zhao et al., GCN Circ. 44787) using the DDRAGO two-channel wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2026-06-03 05:39:27 to 06:02:56 UTC (from 133 seconds to 27 minutes after the trigger) and obtained 1000 seconds of simultaneous exposure in the r/z filters.
The data were reduced, coadded, and analyzed with the COLIBRI ASU pipeline. The photometry was calibrated using nearby stars from the Pan-STARRS-DR2 catalog, is in the AB system, and is not corrected for Galactic extinction.
We detect an uncatalogued source consistent with the ECLAIRs 5.42 arcmin error circle (Zhao et al., GCN Circ. 44787) at:
RA (J2000) = 15:32:34.55 = 233.14397 degrees
Dec (J2000) = +40:01:33.3 = 40.02591 degrees
The preliminary magnitudes derived for this source are:
r = 19.78 +/- 0.01
z = 19.27 +/- 0.01
The source is observed to brighten and then fade. We suggest this is the counterpart of the GRB.
Further observations are ongoing.
We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM.
COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observa
GCN Circular 44787
Subject
GRB 260603A: SVOM detection of a burst
Date
2026-06-03T06:28:01Z (5 months ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
Donghua ZHAO, Wenjin Xie (NAOC), Ruizhi Li (YNAO), Ziming Wang (BNU) report on behalf of the SVOM mission team:
At 2026-06-03T05:38:16 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 260603A (SVOM burst-id sb26060303).
The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network.
The burst was only detected by 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 15.29 in the [5-20] keV energy band over a time window of 81.92 seconds starting at 2026-06-03T05:37:55.
The localization of the best alert is R.A., Dec. 233.1366,40.0801 degrees:
R.A. (J2000) = 15h32m32.79s
Dec. (J2000) = 40d04m48.22s
with a 90% confidence level (C.L.) radius of 5.42 arcmin (including systematic error of 2 arcmin added in quadrature).
One high proper motion star named TYC 3055-1389-1 is at 194.45 arcsec away.
Due to a ToO-EX in performance, no immediate slew was performed on this burst.
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 Ruizhi Li: liruizhi@ynao.ac.cn. Please contact the BA by email if you require additional information.