{
  "eventId": "GRB 260604C",
  "circularId": 44883,
  "subject": "GRB 260604C: SVOM/ECLAIRs refined analysis",
  "createdOn": 1781006287216,
  "submittedHow": "web",
  "submitter": "SVOM_group <svomgroup@bao.ac.cn>",
  "format": "text/plain",
  "body": "M. Brunet, O. Godet (IRAP), N. Dagoneau, D. Gotz, D. Adrien, S. Bisero (CEA) report on behalf of the SVOM/ECLAIRs team:\n\nUsing 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). \n\nThe 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.\n\nThe refined burst localization is RA, Dec = 224.396, 28.742  degrees:\n\nRA (J2000) = 14h57m35.0s\nDec (J2000) = 28d44m31.2s\n\nwith a 90% C.L. radius of 8.6 arcmin (including systematic error of 2 arcmin added in quadrature).\n\nThis localization is consistent with the one of the afterglow (MASTER OT detection, GCN 44827).\n\nThe 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.\n\nThe 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.\n\nFermi/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.\n\nAll the quoted errors are at the 68% confidence level.\n\nThe 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.\n\nThe SVOM/ECLAIRs point of contact for this burst is: M. Brunet (IRAP) (marius.brunet at utoulouse.fr)\n"
}