GCN Circular 40381

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 250507A (Wang et al. GCN 40374
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 ) was observed by SVOM/MXT after an automatic SVOM slew, starting at T0 = 2025-05-07T06:40:59.951 (374s after trigger time Tb). MXT observed during the first orbit for 2.1 ks effective exposure.

Using the full X-band dataset, we measure a complex light curve with two flares of 150 to 200s duration, followed by a rapid decay with a temporal index of -4.1+/-0.2 at times t+T0>400s. The source is no longer detected at t+T0>1200s and hence we restricted the analysis below to the first 20 min of data.

The X-ray source located at R.A., Dec  183.066, -23.5928 degrees:
R.A. (J2000) = 12h12m15.8s
Dec. (J2000) = -23d35m34.12s
with a 90% C.L. radius of 37 arcseconds (including 35 arcseconds systematic error added in quadrature)

The spectrum is modelled with an absorbed power law. Using the average spectrum of the first 1200s of MXT data, the absorption column NH is 5.2 +/-0.5 x 1e22 /cm2 (90% C.L. uncertainties) and a rather soft photon index of 3.6 +/- 0.2. The time-averaged observed flux is 4.5 x 1e-10 ergs/cm^2/s but the count rate varies by a factor 100 between peak at t+T0=160s and t+T0=1200s.

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:z iqi.wang@st.gxu.edu.cn.
Please contact the BA by email if you require additional information regarding the SVOM follow-up of this burst.