TITLE: GCN CIRCULAR NUMBER: 40364 SUBJECT: GRB 250506A: SVOM improved position and MXT detection of a fading X-ray afterglow DATE: 25/05/06 14:47:41 GMT FROM: SVOM_group svomgroup@bao.ac.cn

P. Maggi (ObAS), D. Götz (CEA), H. Goto (Kanazawa University/CEA), M. Moita (CEA), C. Plasse (CEA), F. Robinet (IJCLab), C. Van Hove (IJCLab) on behalf of the SVOM/MXT Team report:

M. Brunet, S. Guillot (IRAP), S. Schanne (CEA) on behalf of the SVOM/ECLAIRs Team report:

Using the full X band dataset a corrected ECLAIRs position could be derived for GRB 250506A/sb25050601 (Wang et al., GCN 40358). The improved ECLAIRs position is RA=219.373, Dec=28.909 with a 90% c.l. of 6.4 arc min.

The burst appeared very close to the Earth limb and only part of the burst signal was considered by the onboard processing, explaining the localization error.

SVOM MXT observed the ECLAIRs error region starting at T0 = 2025-05-06T03:09:28.279 (46 min after trigger time Tb). MXT observed it during 5 orbits for a 10.9 ks effective exposure.

Using the full X band dataset, we find an uncatalogued source. The source is fading with time and not detected after the fourth orbit (t+T0 > 23 ks). The best position is obtained from first orbit data (t+T0 < 2.5ks), at RA=219.267, Dec=28.8593 corresponding to: RA (J2000) = 14h37m04. Dec (J2000) = +28d51m34. with a statistical 90% C.L. radius of 32.5” to which a 35” systematic uncertainty is to be added in quadrature.

The afterglow spectrum is modelled by an absorbed power law fading with time. The absorption column is NH = 3+/-1 x 1e21 /cm2 (90% C.L. uncertainties) and a photon index of 2.3 (+0.7/-0.6) in first orbit softening to 3.3 (+2.5/-1.2) in the second orbit. The average observed flux in the 0.3-8 keV band is 6.8 (+3.4/-2.2) x1e-11 erg/cm2/s during the first orbit and 1.4 (<2.7) x1e-11 erg/cm2/s during the second one.

The ECLAIRs and MXT position are consistent, confirming that the MXT source is the afterglow of the GRB 250506A.

This position corrects and supersedes the one reported by Wang et al. (GCN 40358).

The active QSO detected by TRT (Liu et al., GCN 40360) and Swift XRT (Beardmore et al., GCN 40361) is thus unrelated to GRB 250506A.

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. ECLAIRs was developed jointly by CNES, IRAP, APC and CEA.

The Burst Advocate (BA) on shift for this burst is Chenwei Wang: cwwang@ihep.ac.cn Please contact the BA by email if you require additional information regarding the SVOM follow-up of this burst.

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