GCN Circular 39916

B.-T.Wang(YNAO, CAS), Y.H. Cheng (SWIFAR, YNU), W.J. Xie, D.H. Zhao (NAOC), C.W. Wang (IHEP), and D. Götz (CEA) on behalf of the SVOM mission team.

At 2025-03-29T04:17:54 UTC (Tb) SVOM/ECLAIRs triggered on the gamma-ray burst GRB 250329A (SVOM burst-id sb25032901). The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network.

The burst was detected by both Count-Rate Trigger (CRT) and Image Trigger (IMT), which produced a sequence of 17 alerts. CRT provided the alert with the best signal-to-noise-ratio (SNR) of 37.28 in the [8-50] keV energy band over a time window of 5.10 seconds starting at Tb.

The localization of the best alert is R.A., Dec 197.4374, 9.4849 degrees (J2000) with a 90% C.L.radius of 2.88 arcmin (including systematic error of 2 arcmin added in quadrature).

The burst was also detected by GRM. The GRM light curve shows that this burst consists of a single main pulse. 
The SVOM/GRM lightcurve can be found here:
https://www.bursthub.cn//admin/static/svgrb250329A.png

SVOM slewed to the burst position.

MXT began observing the field at 2025-03-29T04:20:20 UTC, 146 seconds after Tb.

Using onboard processed data we found a faint uncatalogued X-ray source located at R.A., Dec 197.4168, 9.5412 degrees:
RA (J2000) = 13h09m40s
DEC (J2000) = 9d32m28s
with a 90% C.L. radius of 2.5 arc minutes.
This location is 3.59 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 recorded images will be published in a future circular gathering information on the follow-up of the SVOM optical instruments.

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 Boting Wang: wangbaiting@ynao.ac.cn.
Please contact the BA by email if you require additional information regarding the SVOM follow-up of this burst.