GCN 44164: GRB 260321A: SVOM/ECLAIRs refined analysis

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)