GCN Circular 42973

M. Brunet, N. A. Webb  (IRAP), N. Dagoneau, M. Moita (CEA), report on behalf of the SVOM mission team

At 2025-12-03T18:10:27 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 251203B (SVOM burst-id sb25120304).

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

The burst was only detected by the Image Trigger (IMT), which produced a sequence of 9 alerts. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 9.85 in the [5-20] keV energy band over a time window of 81.92 seconds starting at 2025-12-03T18:09:25.

The localization of the best alert is R.A., Dec. 18.4596, -0.0001 degrees (J2000) with a 90% confidence level (C.L.) radius of 8.07 arcmin (including systematic error of 2 arcmin added in quadrature).

SVOM slewed to the burst.

SVOM/MXT began observing the field at 2025-12-03T18:18:55 UTC, 509 seconds after T0. Using onboard processed data we found an uncatalogued X-ray source located at R.A., Dec. 18.3720, 0.0157 degrees:
R.A. (J2000) = 01h13m29.27s
Dec. (J2000) = 00d00m56.42s
with a 90% C.L. radius of 35.36 arcseconds.

This location is 5.34 arcminutes from the ECLAIRs onboard position. This position may be improved as more data is received.

We note the presence of an X-ray source (2E 0110.9-0014) located 20.06 arcsec away from the MXT position. 

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 Marius Brunet: mbrunet@irap.omp.eu.
Please contact the BA by email if you require additional information.