TITLE: GCN CIRCULAR NUMBER: 44883 SUBJECT: GRB 260604C: SVOM/ECLAIRs refined analysis DATE: 26/06/09 11:58:07 GMT FROM: SVOM_group <svomgroup@bao.ac.cn> M. Brunet, O. Godet (IRAP), N. Dagoneau, D. Gotz, D. Adrien, S. Bisero (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 260604C (SVOM burst-id sb26060404 – GCN 44823, trigger time T0 = 2026-06-04T20:21:59 UTC), which was also detected by Fermi/GBM (GCN 44822, 44831) and GECAM-B (GCN 44854). The burst that triggered ECLAIRs onboard shows a multiple peak lightcurve. The burst duration is estimated to be around 255 s in the 4-20 keV energy band through imaging. The refined burst localization is RA, Dec = 224.396, 28.742 degrees: RA (J2000) = 14h57m35.0s Dec (J2000) = 28d44m31.2s with a 90% C.L. radius of 8.6 arcmin (including systematic error of 2 arcmin added in quadrature). This localization is consistent with the one of the afterglow (MASTER OT detection, GCN 44827). The time-averaged spectrum, before the slew (from T0-56.3 s to T0+41.9 s) in the 5-120 keV energy range is best fitted by a powerlaw model with alpha = -2.02 +0.17/-0.19. With this model, the 4-120 keV fluence is (4.5 +0.5/-1.3)e-7 erg/cm² and the 4-120 keV photon flux is 0.21 +0.01/-0.05 ph/cm²/s. The time-averaged spectrum after the slew (from T0+100 s to T0+200 s) in the 5-120 keV energy range is best fitted by a powerlaw model with alpha = -2.09 +0.35/-0.48. With this model, the 4-120 keV fluence is (2.2 +0.2/-1.3 )e-7 erg/cm² and the 4-120 keV photon flux is 0.10 +0.01/-0.05 ph/cm²/s. Fermi/GBM trigger occurred at T0-195 s at a time when ECLAIRs was not collecting data. So ECLAIRs missed the main part of the burst prompt emission. The spectral parameters we obtained may indicate that ECLAIRs observed the transition from the prompt to afterglow emission. 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: M. Brunet (IRAP) (marius.brunet at utoulouse.fr) View this GCN Circular online at https://gcn.nasa.gov/circulars/44883. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...