GCN Circular 41368
Subject
Subject GRB 250813B: SVOM/MXT refined analysis
Event
Date
2025-08-14T11:45:54Z (10 days ago)
From
Diego Gotz at CEA <diego.gotz@cea.fr>
Via
Web form
D. Götz (CEA), F. Robinet (IJCLab), P. Maggi (ObAS), H. Goto (Kanazawa Univ./CEA), M. Moita (CEA), C. Plasse (CEA), , C. Van Hove (IJCLab) report of behalf of the SVOM/MXT Team:
GRB 250813B (Xie et al. GCN 41352/41354) was observed by SVOM/MXT after an automatic SVOM slew, starting at T0 = 2025-08-13T22:54:01, 169 s after trigger time Tb. MXT observed for the remainder of the orbit, cumulating 188 s of useful data before entering Earth occultation.
Using the full X-band dataset of the first orbit, the position of the MXT afterglow is refined to:
R.A. (J2000) = 22h26m58.6s
Dec (J2000) = +12d27m08s
with a 90% C.L. radius of 54 arc sec (including 25 arcseconds systematic error added in quadrature). This position is at 47 arc sec from the optical counterpart reported in GCN 41353.
The light curve exhibits an initial fast fading with temporal decay of -2.3+/-0.5.
We analysed the early time (first orbit) spectrum and modelled it with a redshifted (z=1.75 GCN 41363) absorbed power-law, a photon index Gamma 1.4+-/0.3 at 90% C.L and a redshifted absorbing column of NH = <0.8x1e22 cm^-2 on top of a Galactic NH = 4.86x10^20 cm^-2.
The source is still detected on the subsequent orbit between Tb+3900 and Tb+6100 s but not after.
The Space 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. MXT was developed jointly by CEA, CNES, University of Leicester, IJCLab and MPE.
The SVOM point of contact for this burst is Wenjin Xie (xiewj@bao.ac.cn).
Please contact the BA by email if you require additional information regarding the SVOM follow-up of this burst.