GRB 260321A — All Circulars

M. Brunet, O.Godet, F. Triot, J.-L. Atteia (IRAP), 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 260321A (SVOM burst-id sb26032123 – GCN 44071, trigger time T0 = 2026-03-21T18:12:08 UTC), which was also detected by Fermi/GBM as a sub-threshold event (GCN 44072). 

The burst that triggered ECLAIRs onboard shows a single peak lightcurve, followed by a featureless emission tail. The burst duration is estimated to be around 57 s in the 4-50 keV energy band through imaging.

The ECLAIRs light curve can be found here:
https://docs.google.com/document/d/1aA9Ams6NNpz7RKQksAyu6mulcFsYo2OPOXFnpyykcok

The time-averaged spectrum from T0-16 s to T0+34.92 s in the 5-120 keV energy range is best fitted by a broken power-law model with a first photon index of -0.65 +0.32/-0.25, a second photon index of -2.71 +0.77/-1.59 and a break energy of 33.4 +4.6/-4.7 keV. With this model, the 4-120 keV fluence is  (4.9 +0.6/-3.6)e-7 erg/cm^2 and the 4-120 keV photon flux is 0.23 +0.20/0.17 ph/cm^2/s. 

This GRB shows some spectral evolution. The spectrum from T0-16s s to T0 in the 5-120 keV energy range is best fitted by a power-law model with a photon index of -1.06 +/-0.13. With this model, the 4-120 keV flux is  (2.3 +0.1/-0.6)e-8 erg/cm²/s. 

The spectrum from T0 to T0+34.92 s in the 5-120 keV energy range is well fitted using a broken power-law model, fixing the first photon index to -1 and the second to -3, with a break energy of 35.7 +12.6/-10.4 keV. With this model, the 4-120 keV flux is (0.7 +/-0.1)e-9 erg/cm²/s.

The position of GRB 260321A in the Amati relation diagram is compatible with those of Type-II GRBs if it is at a redshift larger than 3. The non detection of an optical afterglow even with early deep observations (SVOM/Colibri upper limit, GCN 44078 & GCN 44102 and SVOM/VT upper limit GCN 44080) may also favor a high redshift origin for this burst.

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)

Antonio de Ugarte Postigo (LAM), Fredd Sánchez Álvarez (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), 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), Noémie Globus (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM) SVOM BA1 and SVOM BA2 report:

We imaged the field of the SVOM GRB 260321A (Brunet et al., GCN Circ. 44071) using the DDRAGO wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2026-03-22
06:35:41 to 09:32:17 UTC (from 12.39 to 15.34 hours after the trigger) and obtained 128 minutes of exposure in the r and z filters. A second comparison epoch was observed from 2026-03-24 06:16:18
 to 09:07:59 UTC (from 60.07 to 62.93 hours after the trigger) again in r and z with 128 min of exposure.

The data were reduced and coadded with the ASU pipeline, and analyzed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.

In the comparison and subtraction of the two epochs, we do not detect any credible new source at any of the FXT source positions (Zou et al., GCN Circ. 44086) down to the following 3-sigma limit:

r > 24.7
z > 23.6

These upper limits supersede the ones reported by de Ugarte Postigo et al. (GCN Circ. 44078).

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.

Z.-C. Zou, Y. H. Jiang (NJU), J. P. Chen (SYSY), Y. J. Yi (BNU), W. D. Zhang (NAOC, CAS) on behalf of the Einstein Probe (EP) team:

EP-FXT performed a follow-up observation of the SVOM/ECLAIRs-detected burst GRB 260321A (SVOM/sb26032123; Brunet et al., GCN 44071) starting at 2026-03-22T15:23:32 UTC, approximately 21 hours after the SVOM trigger, with a total exposure time of 4565 s. Four uncataloged sources are detected within the ECLAIRs error circle. Preliminary analysis on these sources is automatically conducted, and the details are listed as follows.

EPF_J143847.8+314542
RA (J2000): 219.6990
Dec (J2000): 31.7616
Flux: 9.265 x 10^-14 erg/s/cm2 (observed, 0.5-10 keV)
Flux_err: 3.498 x 10^-14 erg/s/cm2 (1 sigma)

EPF_J143759.7+314425
RA (J2000): 219.4989
Dec (J2000): 31.7404
Flux: 2.9252 x 10^-14 erg/s/cm2 (observed, 0.5-10 keV)
Flux_err: 1.2651 x 10^-14 erg/s/cm2 (1 sigma)

EPF_J143833.9+314628
RA (J2000): 219.6440
Dec (J2000): 31.7719
Flux: 5.9349 x 10^-14 erg/s/cm2 (observed, 0.5-10 keV)
Flux_err: 2.0193 x 10^-14 erg/s/cm2 (1 sigma)

EPF_J143845.7+313613
RA (J2000): 219.6902
Dec (J2000): 31.6037
Flux: 8.0698 x 10^-14 erg/s/cm2 (observed, 0.5-10 keV)
Flux_err: 1.9625 x 10^-14 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).

Z.-C. Zou, Y. H. Jiang (NJU), J. P. Chen (SYSY), Y. J. Yi (BNU), W. D. Zhang (NAOC, CAS) on behalf of the Einstein Probe (EP) team:

EP-FXT performed a follow-up observation of the SVOM/ECLAIRs-detected burst GRB 260321A (SVOM/sb26032123; Brunet et al., GCN 44071) starting at 2026-03-22T15:23:32 UTC, approximately 21 hours after the SVOM trigger, with a total exposure time of 4565 s. Four uncataloged sources are detected within the ECLAIRs error circle. Preliminary analysis on these sources is automatically conducted, and the details are listed as follows.

EPF_J143847.8+314542
RA (J2000): 219.6990
Dec (J2000): 31.7616
Flux: 9.265 x 10^-14 erg/s/cm2 (observed, 0.5-10 keV)
Flux_err: 3.498 x 10^-14 erg/s/cm2 (1 sigma)

EPF_J143759.7+314425
RA (J2000): 219.4989
Dec (J2000): 31.7404
Flux: 2.9252 x 10^-14 erg/s/cm2 (observed, 0.5-10 keV)
Flux_err: 1.2651 x 10^-14 erg/s/cm2 (1 sigma)

EPF_J143833.9+314628
RA (J2000): 219.6440
Dec (J2000): 31.7719
Flux: 5.9349 x 10^-14 erg/s/cm2 (observed, 0.5-10 keV)
Flux_err: 2.0193 x 10^-14 erg/s/cm2 (1 sigma)

EPF_J143845.7+313613
RA (J2000): 219.6902
Dec (J2000): 31.6037
Flux: 8.0698 x 10^-14 erg/s/cm2 (observed, 0.5-10 keV)
Flux_err: 1.9625 x 10^-14 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).

H. L. Li, Y. N. Ma, C. Wu,  Z. H. Yao,  Y. L. Qiu, L. P. Xin, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. R. Xu, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA/Irfu), M. Brunet (IRAP), O. Godet (IRAP)  report on behalf of the SVOM/VT team.

SVOM/VT performed an observation with automatic slew to the field of GRB 260321A triggered by SVOM/ECLAIRs (sb26032123, Brunet et al., GCN 44071). The observation started at 2026-03-21T18:57:18 UTC, i.e., about 45 minutes post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously. 

No credible sources were detected within the error box of ECLAIRs (Brunet et al., GCN 44071), compared to the Legacy survey catalog. The 5 sigma upper limits in AB magnitude are derived as follows:

Mid time  | Band | Exposure Time | 5 sigma upper limit
1.13 hour   VT_B      53*50 sec        > 22.7 mag    
1.12 hour   VT_R      42*50 sec        > 22.5 mag

More detailed analysis is still ongoing. 

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.

Antonio de Ugarte Postigo (LAM), Edilberto Aguilar-Ruiz (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (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), Noémie Globus (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), M. Brunet, O. Godet (IRAP) report:

We imaged the field of the SVOM GRB 260321A (Brunet et al., GCN Circ. 44071) using the DDRAGO wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2026-03-22 06:35:41 to 07:59:54 UTC (from 12.39 to 13.80 hours after the trigger) and obtained 61 minutes of exposure in the r and z filters.

The data were reduced and coadded with the COLIBRÍ pipeline and analyzed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.

In the stacked image, we do not detect any new source within the ECLAIRs source position (Brunet et al., GCN Circ. 44071), as compared to the Legacy Survey, down to the following 5-sigma limit:

r >  23.7 mag
z > 22.6 mag

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 Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.

R. Hamburg (USRA) reports on behalf of the Fermi-GBM Team:

SVOM/ECLAIRs detected GRB 260321A on 2026-03-21 at 18:12:08 UTC (GCN 44071). There was no Fermi-GBM onboard trigger around this event time. An automated, blind search for gamma-ray bursts below the onboard triggering threshold in Fermi-GBM identified no candidates.

The GBM Targeted Search [1], the most sensitive coherent search for GRB-like signals in GBM, identified a transient approximately 10 s after the ECLAIRs best image SNR time of 2026-03-21T18:11:48. The Targeted Search candidate was found most significantly on the 8 s timescale using the "normal" spectral template (i.e., Band function with Epeak = 230 keV, alpha = -1.0, beta = -2.3) and has a false alarm rate of 3.7e-04 Hz. The Fermi-MET of this transient is 795809523.353 s. The Targeted Search localization is spatially consistent with the ECLAIRs location.
 
[1] Goldstein et al. 2019 arXiv:1903.12597

M. Brunet, O. Godet (IRAP) report on behalf of the SVOM mission team:

At 2026-03-21T18:12:08 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 260321A (SVOM burst-id sb26032123).

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 2 alerts. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 9.37 in the [8-120] keV energy band over a time window of 40.96 seconds starting at 2026-03-21T18:11:48.

The localization of the best alert is R.A., Dec. 219.574, 31.674 degrees:
R.A. (J2000) = 14h38m17.65s
Dec. (J2000) = 31d40m28.18s
 with a 90% confidence level (C.L.) radius of 8.45 arcmin (including systematic error of 2 arcmin added in quadrature).

This burst also triggered SVOM/GRM at 2026-03-21T18:12:08 with an SNR of 10.9.
The SVOM/GRM light curve showed a multiple peak structure with a T90 duration of about 113.02 -18.24 / +74.15 (8-1100 keV).

SVOM slewed to the burst.

No X-ray observation could be performed by SVOM/MXT for the time being. No optical observation could be performed by SVOM/VT for the time being.

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 Marius Brunet: mbrunet@irap.omp.eu.
Please contact the BA by email if you require additional information.