TITLE: GCN CIRCULAR NUMBER: 44675 SUBJECT: GRB 260509A: SVOM/ECLAIRs refined analysis DATE: 26/05/20 07:09:29 GMT FROM: SVOM_group <svomgroup@bao.ac.cn> F. Daigne (IAP), F. Lacreu (IAP), U. Jacob (LUPM), M. Brunet (IRAP), D. Götz (CEA/Irfu), A. Saccardi (CEA/Irfu), H. Goto (RIKEN), D. Adrien (CEA) 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 260509A (SVOM burst-id sb26050905). The burst that triggered ECLAIRs onboard at T0 = 2026-05-09T21:51:09 UTC (GCN 44504) shows a multiple peaks lightcurve with a precursor starting at about T0-22 s for about 8 s. The burst duration is T90 = 14.3 +0.9/-1.3 s in the 4-120 keV energy band. The time-averaged spectrum from T0-1.4 s to T0+12.9 s in the energy range 5-120 keV is best fitted by a power-law model with a photon index -2.20 +/- 0.09. With this model, the 4-120 keV fluence is (4.1 +/- 0.3)E-7 erg/cm^2 and the 4-120 keV photon flux is 1.49 +/- 0.07 ph/cm^2/s. The value of the photon index indicates that this GRB is very soft and should probably be classified as an X-Ray Flash. The time-averaged spectrum of the precursor, from T0-22 s to T0-14.5 s in the energy range 5-20 keV is best fitted by a power-law model with a photon-index -3.0 +0.5/-0.6. It can also be fitted with a blackbody with temperature kT = 1.5 +0.4/-0.3 keV. 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: F. Daigne (IAP) (daigne at iap.fr) View this GCN Circular online at https://gcn.nasa.gov/circulars/44675. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...