TITLE: GCN CIRCULAR NUMBER: 39925 SUBJECT: GRB 250329B: SVOM detection of a faint transient DATE: 25/03/29 13:24:22 GMT FROM: SVOM_group svomgroup@bao.ac.cn

W.J. Xie (NAOC), Y.H. Cheng (SWIFAR, YNU), B.T. Wang (YNAO, CAS), O. Godet, S. Guillot (IRAP), S. Schanne (CEA), M. Brunet (IRAP) on behalf of the SVOM mission team.

SVOM/ECLAIRs triggered on the gamma-ray burst GRB 250329B (SVOM burst-id sb25032902) starting at 2025-03-29T12:10:18 UTC (Tb).

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

The burst was detected by the Image Trigger (IMT), which produced a sequence of 2 alerts. IMT provided the alert with the best signal-to-noise-ratio (SNR) of 7.33 in the 8-120 keV energy band over a time window of 40.96 seconds starting at Tb.

The localization of the best alert is R.A., Dec 207.6731, 52.8853 degrees: RA (J2000) = 13h50m41.55s Dec (J2000) = 52d53m07.02s with a 90% C.L. radius of 10.69 arcmin (including systematic error of 2 arcmin added in quadrature).

No immediate slew was performed on this burst, because its significance was below the slew threshold. A SVOM ToO will be performed.

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.

The Burst Advocate (BA) on shift for this burst is Wenjin XIE: xiewj@bao.ac.cn. Please contact the BA by email if you require additional information regarding the SVOM follow-up of this burst.

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