TITLE: GCN CIRCULAR NUMBER: 41243 SUBJECT: GRB 250806A: SVOM detection of a soft burst DATE: 25/08/06 08:39:15 GMT FROM: SVOM_group svomgroup@bao.ac.cn

W. J. Xie (NAOC), L. P. Xin (NAOC), S. Schanne (CEA), L. Bouchet (IRAP), L. Zhang (IHEP), H. Goto (Kanazawa Univ.), P. Maggi (ObAS), report on behalf of the SVOM mission team:

At 2025-08-06T07:58:14 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 250806A (SVOM burst-id sb25080601). 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 15 alerts. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 10.48 in the 5-20 keV energy band over a time window of 20.48 seconds starting at 2025-08-06T07:58:04 (Tb).

The localization of the best alert is R.A., Dec. 348.4237, 1.3791 degrees (J2000) with a 90% confidence level (C.L.) radius of 7.62 arcmin (including systematic error of 2 arcmin added in quadrature). The transmitted ECLAIRs VHF light curve shows a single peak of a duration of about 20 seconds mainly in 5-20 keV and the GRM VHF light curve shows no clear signal. Future X-band data will provide more information on the burst duration and spectrum.

SVOM slewed to the burst.

SVOM/MXT began observing the field at 2025-08-06T08:00:11 UTC, 117 seconds after T0. Using onboard processed data we found an uncatalogued X-ray source located at R.A., Dec. 348.4214, 1.3820 degrees:

R.A. (J2000) = 23h13m41.1s Dec. (J2000) = 01d22m55s

with a 90% C.L. radius of 85 arcseconds (including a systematic uncertainty of 25” added in quadrature).

This location is 0.22 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 Wenjin Xie (xiewj@bao.ac.cn). Please contact the BA by email if you require additional information.

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