GCN Circular 39888

Laurent Bouchet, Hui Yang, Olivier Godet, Marius Brunet (IRAP), Clara Plasse (CEA)
on behalf of the SVOM mission team.

SVOM/ECLAIRs triggered on the gamma-ray burst GRB 250327B (SVOM burst-id sb25032706) starting at 2025-03-27T21:11:27 UTC (Tb). 
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 10 alerts. IMT provided the alert with the best signal-to-noise-ratio (SNR) of 12.05 in the [8-120] keV energy band over a time window of 40.96 seconds starting at Tb. The light curve shows that the burst duration is about 300 s and the burst is still detected after the slew. 

The localization of the best alert is R.A., Dec 176.7763, 29.8765 degrees (J2000) with a 90% C.L. radius of 6.70 arcminutes (including systematic error of 2 arcminutes added in quadrature).

SVOM slewed to the burst.

MXT began observing the field at 2025-03-27T21:14:41 UTC, 193 seconds after Tb.

Using onboard processed data we found an uncatalogued X-ray source located at R.A., Dec 176.788, 29.832 degrees:
RA (J2000) = 11h47m09.15s
DEC (J2000) = 29d49m55.57s
with a 90% C.L. radius of 25 arcseconds.
This MXT location is within the ECLAIRs error circle. 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 Hui Yang: hui.yang@irap.omp.eu.
Please contact the BA by email if you require additional information regarding the SVOM follow-up of this burst.