GCN Circular 42437

S. Hussein (IJCLab), M. Brunet (IRAP), U. Jacob (LUPM), D. Gotz (CEA), P. Maggi (ObAS), report on behalf of the SVOM mission team:

At 2025-10-25T14:24:05 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 251025B (SVOM burst-id sb25102502).

The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network.

The burst was detected both by the Count-Rate Trigger (CRT) and the Image Trigger (IMT), which produced a sequence of 13 alerts. CRT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 17.67 in the [8-120] keV energy band over a time window of 10.20 seconds starting at 2025-10-25T14:24:00.

The localization of the best alert is R.A., Dec. 29.5647, 30.2424 degrees (J2000) with a 90% confidence level (C.L.) radius of 4.80 arcmin (including systematic error of 2 arcmin added in quadrature).

This burst was also detected by SVOM/GRM with a significance of 8.80.
SVOM slewed to the burst.

SVOM/MXT began observing the field at 2025-10-25T14:27:11 UTC, 186 seconds after T0. Using onboard processed data we found an uncatalogued X-ray source located at R.A., Dec. 29.542, 30.308 degrees:
R.A. (J2000) = 1h58m10.23s
Dec. (J2000) = 30d18m32.2s
with a 90% C.L. radius of 127.48 arcseconds.

This location is 4.15 arcminutes from the ECLAIRs onboard position. This position may be improved as more data is received.

VT began observing the field after the slew. The analysis of the data will be published in a future circular.

The Space-based multi-band astronomical 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. SVOM/ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. SVOM/GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. SVOM/MXT was developed jointly by CNES, CEA-IRFU, CNRS-IJCLab, University of Leicester, MPE.

The Burst Advocate (BA) on shift for this alert is Shaymaa Hussein: hussein@astro.uni-tuebingen.de.
Please contact the BA by email if you require additional information.