GCN Circular 41026
Subject
GRB 250706B: SVOM observations interpreted as the afterglow of GRB 250706C
Event
Date
2025-07-08T13:38:21Z (3 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
J. T. Palmerio, D. Adrien (CEA), M. G. Bernardini (INAF-OAB, LUPM), M. Brunet, J.-L. Atteia, O. Godet (IRAP), P. Maggi (ObAS), L. P. Xin (NAOC), D. B. Malesani (DAWN/NBI and Radboud), C. W. Wang (IHEP), A. Saccardi, D. Götz (CEA), F. Cangemi (APC) report on behalf of the SVOM mission team:
Following the detection of GRB 250706C reported by Konus-Wind (Frederiks et al., GCN 41013) and Fermi/LAT (Longo et al., GCN 41019), we discuss below the interpretation of GRB 250706B detected by SVOM (Palmerio et al., GCN 4989) as the hard X-ray afterglow of GRB 250706C.
At the Konus-Wind trigger time (Frederiks et al., GCN 41013), the position of GRB 250706B was occulted by the Earth for SVOM/ECLAIRs. The SVOM/ECLAIRs trigger for GRB 250706B occurred 21 min after the Konus-Wind trigger, right after the source was no longer occulted by the Earth. SVOM/ECLAIRs shows a featureless and long-lasting (~1000 s) emission in the 4-50 keV energy band. The time-averaged spectrum after the slew (from 134 s to 1000 s after the ECLAIRs trigger time, GCN 4989) in the energy range 5-120 keV is best fitted by a power law model with index 2.01 +/- 0.11 (68% c.l. errors). With this model, the total 4-120 keV fluence is 2.6e-6 erg/cm^2. There is no significant signal in the SVOM/GRM VHF data.
Taking the Konus-Wind detection time as T0 (16:45:22 UT), the SVOM/VT lightcurve appears as a single, unbroken power-law with index ~1.2. Assuming the same power-law decay held continuously before the VT observations, extrapolating the optical light curve to T0 + 1 min yields a magnitude of roughly 10 in VT_R. We encourage all-sky monitors to check for any emission at this location around the Konus-Wind trigger time.
Extrapolating the SVOM/MXT lightcurve to T0 + 1 min yields a 0.3-10 keV flux of 6.5x10^-8 erg/cm^2/s, assuming the same power-law decay held continuously before the MXT observations reported by Maggi et al. (GCN 41009).
We encourage further follow-up of this burst.
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. SVOM/ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. SVOM/GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. SVOM/MXT was developed jointly by CNES, CEA-IRFU, CNRS-IJCLab, University of Leicester, MPE.
The SVOM point of contact for this burst is Jesse Palmerio: palmerio@cea.fr
Please contact the BA by email if you require additional information regarding the SVOM follow-up of this burst.