GCN Circular 41288
Subject
GRB 250806A: SVOM/MXT refined analysis
Event
Date
2025-08-08T14:08:03Z (16 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
P. Maggi (ObAS), D. Götz (CEA), H. Goto (Kanazawa University/CEA), M. Moita (CEA), C. Plasse (CEA), F. Robinet (IJCLab), C. Van Hove (IJCLab) report of behalf of the SVOM/MXT Team:
GRB 250806A (Xie et al. GCN 41243) was observed by SVOM/MXT after an automatic SVOM slew, starting at T0 = 2025-08-06T08:00:11, 117 s after trigger time Tb. MXT observed for the remainder of the orbit for 297s.
Using the full X-band dataset, the position of the MXT candidate afterglow is refined to:
R.A. (J2000) = 23h13m38.10s
Dec (J2000) = +01d21m59.0s
with a 90% C.L. radius of 66” (including 25 arcseconds systematic error added in quadrature). This is just outside the Swift/XRT afterglow identified by Evans et al. (GCN 41249) and observed with Einstein Probe/FXT (Liang et al., GCN 41260), but the light curve analysis below confirms that this is the same source.
We analysed the time-averaged spectrum, subtracting an appropriately-scaled blank sky background spectrum from the Lockmann Hole. Modelled with an absorbed power-law, a soft spectrum is found, with a photon index Gamma >2.2 at 90% C.L and an absorbing column NH = 4.1 (+4.5/-3.2) x1e21 cm2 on top of Galactic NH = 4.6 x 1e20 /cm2.
We derive a count rate conversion factor of 1 cps = 9.9e-11 erg/s/cm2
The light curve exhibits an initial fast decay with temporal index alpha~-3.5+/-0.6 (with count rate proportional to t^alpha). At t >Tb + 250s the source is no longer detected by MXT. Extending the light curve using Swift/XRT data in Photon Counting mode from the automatic follow-up suggests a transition to a shallower decay occurred at about 5 minutes post-trigger.
The source is below the MXT detection limit in subsequent orbits.
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.