TITLE: GCN CIRCULAR NUMBER: 43953 SUBJECT: GRB 260307B: SVOM/ECLAIRs refined analysis DATE: 26/03/10 16:09:19 GMT FROM: SVOM_group <svomgroup@bao.ac.cn> M. Brunet, L. Bouchet (IRAP), F. Magnani, A. Foisseau (CPPM), 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 260307B (SVOM burst-id sb26030704 – GCN 43940, trigger time T0 = 2026-03-07T13:45:26 UTC), which was also detected by Fermi/GBM (GCN 43948). The burst that triggered ECLAIRs shows a single peak lightcurve lasting for around 41 s prior to the slew, followed by a featureless emission after the slew. The burst duration is estimated to be around 580 s in the 4-120 keV energy band through imaging. The time-averaged spectrum before the slew (from T0-24 s to T0+17 s) in the energy range 5-120 keV is best fitted by a powerlaw model with a photon index of -1.29 +/-0.10. With this model, the 4-120 keV fluence is (9.4 +0.5/-1.4)e-7 erg/cm^2 and the 4-120 keV photon flux is 0.54 +0.02/-0.06 ph/cm^2/s. The time-averaged spectrum after the slew (from T0+151 s to T0+560 s) in the energy range 5-120 keV is best fitted by a powerlaw model with an exponential cutoff. The photon index is -0.96 +0.37/-0.33 and the peak energy is 36 +38/-14 keV. With this model, the 4-120 keV fluence is (1.4 +0.1/-0.7)e-6 erg/cm^2 and the 4-120 keV photon flux is 0.12 +0.01/-0.06 ph/cm^2/s. Using the best fit parameter from the spectrum before the slew to fit the spectrum after the slew does not give a good fit (chi²/dof: 21.1/12), which implies the presence of a spectral evolution with time. 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) View this GCN Circular online at https://gcn.nasa.gov/circulars/43953. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...