GCN Circular 44236

M. Brunet, O.Godet (IRAP), W.-J. Xie (NAOC), W. Tan, R. Li (IHEP), 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 260402A(SVOM burst-id sb26040203– GCN 44181
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 , trigger time T0 = 2026-04-02T10:30:17 UTC), which was also detected by EP/WXT (GCN 44186
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 ). 

The burst that triggered ECLAIRs onboard shows a multiple peak lightcurve. The burst duration is T90 = (15.2 -3.9/+3.1) s in the 4-120 keV energy band. However, some emission is seen for 41 s from T0 up to T0 + 41 s in the 4-20 keV energy band through imaging.

The time-averaged spectrum from T0 to T0+41s in the energy range 5-120 keV is best fitted by a powerlaw model with a photon index of -2.18 +0.09/-0.10. With this model, the 4-120 keV fluence is  (5.40 +0.33/-0.57)e-7 erg/cm^2 and the 4-120 keV photon flux is 0.67 +0.03/-0.04 ph/cm^2/s. 

This GRB shows some spectral evolution.

The spectrum from T0 to T0 + 1 s in the 5-120 keV energy range is best fitted by a power-law model with a photon index of -1.37 +/-0.12. With this model, the 4-120 keV flux is (8.9 +0.6/-1.7)e-8 erg/cm²/s.

The spectrum from T0 + 1 s to T0 + 5 s in the 5-120 keV energy range is best fitted by a broken power-law model with the second photon index fixed at -3, which gives a first photon index of -0.85 +0.23/-0.20 and a break energy of 17.3 +2.0/-1.6 keV. With this model, the 4-120 keV flux is (4.6 +0.1/-1.1)e-8 erg/cm²/s.

The spectrum from T0 + 5 s to T0 + 10 s in the 5-120 keV energy range is best fitted by a broken power-law model with the first photon index fixed at -0.85 and the second one fixed at -3, which gives a break energy of 8.6 +1.2/-0.8 keV. With this model, the 4-120 keV flux is (2.3 +0.1/-0.2)e-8 erg/cm²/s.

The spectrum from T0 + 10 s to T0 + 41 s in the 5-50 keV energy range is best fitted by a power-law model with a photon index of -2.9 +0.3/-0.4. With this model, the 4-120 keV flux is (4.8 +0.3/-2.0)e-9 erg/cm²/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) (mbrunet at utoulouse.fr)