TITLE: GCN CIRCULAR NUMBER: 43000 SUBJECT: SVOM/sb25120502: SVOM detection of a faint X-ray transient candidate DATE: 25/12/05 13:40:22 GMT FROM: SVOM_group svomgroup@bao.ac.cn

W. J. Xie, D. H. Zhao (NAOC), S. Schanne (CEA/Irfu), Marius Brunet, J.-L. Atteia (IRAP) report on behalf of the SVOM mission team:

At 2025-12-05T02:57:15 UTC (T0), SVOM/ECLAIRs triggered and located a possible X-ray transient (SVOM burst-id sb25120502).

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

The transient was only detected by the Image Trigger (IMT), which produced a sequence of 1 alert. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 6.58 in the [8-50] keV energy band over a time window of 20.48 seconds starting at 2025-12-05T02:56:55.

After the reception over X-band of the photon by photon data, those have been replayed on ground in the Offline trigger as well as a ground copy of the On-board trigger, by varying the trigger timescales. The event is still detected as a point-like source at the same position in the same energy band as given by the on-board trigger, over the same 20.48 s time-window. The sky images obtained are clean and show no sign of the source before and after that time period.

The localization of the best alert is R.A., Dec. 80.9924, -18.2171 degrees: R.A. (J2000) = 5h23m58.16s Dec. (J2000) = -18d13m01.48s with a 90% confidence level (C.L.) radius of 11.88 arcmin (including systematic error of 2 arcmin added in quadrature).

No immediate slew was performed on this burst. No X-ray observation could be performed by SVOM/MXT for the time being. No optical observation could be performed by SVOM/VT for the time being.

We notice the presence of the High Proper Motion star LP 777-23, located at 9.3 arcmin from the trigger 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 contact point for this alert is Wenjin XIE: xiewj@bao.ac.cn Please contact by the email if you require additional information.

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