TITLE: GCN CIRCULAR NUMBER: 41405 SUBJECT: GRB 250818B: SVOM detection of a short burst DATE: 25/08/18 04:00:21 GMT FROM: SVOM_group svomgroup@bao.ac.cn
Z. Q. Wang (GXU), Y. F. Liang (PMOC), W. J. Xie, D. H.Zhao (NAOC), F. Robinet (IJCLab), L. Zhang (IHEP), report on behalf of the SVOM mission team:
At 2025-08-18T03:29:09 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 250818B (SVOM burst-id sb25081801).
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 8 alerts. CRT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 10.74 in the [5-20] keV energy band over a time window of 2.50 seconds starting at 2025-08-18T03:29:08.
The localization of the best alert is R.A., Dec. 46.0666, -3.1784 degrees: R.A. (J2000) = 3h04m15.99s Dec. (J2000) = -3d10m42.10s with a 90% confidence level (C.L.) radius of 7.44 arcmin (including systematic error of 2 arcmin added in quadrature).
This burst also triggered SVOM/GRM at 2025-08-18T03:29:08 on a timescale of 1 seconds with an SNR of 6.80. https://www.bursthub.cn//admin/static/svgrb250818B.png
SVOM slewed to the burst. However, the MXT was occulted by the Earth. The burst localization results will be published later in another GCN circular.
A SVOM ToO has been programmed for follow-up.
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 Ziqi.Wang: ziqi.wang@st.gxu.edu.cn. Please contact the BA by email if you require additional information.
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