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GCN Circular 41009

Subject
GRB 250706B: refined SVOM/MXT data analysis
Event
GRB 250706B
Date
2025-07-07T15:24:57Z (12 hours 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 250706B (Palmerio et al. GCN 40989Loading...
 
 ) was observed by SVOM/MXT after an automatic SVOM slew, starting at T0 = 2025-07-06T17:08:42.6 (158 s after trigger time Tb). MXT observed during 6 orbits for 13.6 ks effective exposure time.

Using the full X-band dataset, the X-ray source afterglow is located at R.A., Dec = 41.239, -50.065 degrees:
R.A. (J2000) = 02h44m57.45s
Dec. (J2000) = -50d03m55.6s
with a 90% C.L. radius of 31 arcseconds (including 30 arcseconds systematic error added in quadrature) and consistent with the reported optical VT position (Palmerio et al. GCN 40992Loading...
 
 ).

The spectrum is modelled with an absorbed power law. Across the full data set the absorption column NH is consistent within uncertainties, between 2 and 3 x 1e22 /cm2 (all uncertainties are at the 90% C.L.). The photon index evolves from 2.25 (-0.16/+0.18) within 1200s of trigger time, to 2.53 (-0.25/+0.28) at Tb + 5 ks < t < Tb+ 20 ks, and softens further to 3.43 (-1.04/+1.45) at t > Tb + 25 ks. The 0.3-10 keV flux is 8.24e-10 (-0.91/+0.82) x 1e-10 erg/cm2/s, 8.9 (-1.0/+1.2) x 1e-11 erg/cm2/s, and down to <1.0 x 1e-11 erg/cm2/s during the same intervals.

We observe a complex light curve with at least 3 power-law segments with count rate proportional to t^alpha, with:
alpha = -0.42 +/-0.02 until Tb+5031s +/-277s
alpha = -1.57 +/-0.07 until Tb+24326s +/-3479s
alpha = -3.05 +/-1.62 afterwards
The SVOM/MXT light curve can be found here: https://seafile.unistra.fr/f/909c01d644774417897e/. With the spectrum reported in the first interval, a count rate of 1 count/s corresponds to a flux of 1.89x1e-10 erg/cm2/s.

Follow-up observations at other wavelengths are encouraged.

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 Jesse Palmerio: palmerio@cea.fr
Please contact the BA by email if you require additional information regarding the SVOM follow-up of this burst.