GCN Circular 41677

Tais Maiolino (LUPM), Claire Guépin-Detrigne (LUPM), Nicolas Dagoneau (CEA/Irfu), Stéphane Schanne (CEA/Irfu), Miguel Moita (CEA/Irfu) report on behalf of the SVOM mission team:

At 2025-09-03T17:24:56 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 250903A (SVOM burst-id sb25090304).

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 12 alerts. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 16.17 in the [5-20] keV energy band over a time window of 20.48 seconds starting at 2025-09-03T17:24:43.

The localization of the best alert is R.A., Dec. 356.9689, -75.9460 degrees:
R.A. (J2000) = 23h47m52.54s
Dec. (J2000) = -75d56m45.52s
with a 90% confidence level (C.L.) radius of 5.17 arcmin (including systematic error of 2 arcmin added in quadrature).

This burst was also detected by SVOM/GRM with a significance of 15.80.
The SVOM/GRM light curve showed a multi-peak structure with a T90 duration of about 27 (-11/+4)s.

SVOM slewed to the burst.

No X-ray observation has been performed by SVOM/MXT for the time being.
No optical observation has been performed by SVOM/VT for the time being.

We notice the presence of BLLac 2FGL J2351.6-7558 located at 12 arcmin from the ECLAIRs position.

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 Tais Maiolino: tais.maiolino@lupm.in2p3.fr.
Please contact the BA by email if you require additional information.