GCN Circular 40439

Pierre Maggi (ObAS), Yinuo Ma (NAOC), Nicolas Dagoneau (CEA), Ulysse Jacob (LUPM), Miguel Moita (CEA) report on behalf of the SVOM mission team:

SVOM/ECLAIRs triggered and located GRB 250512B (SVOM burst-id sb25051203) starting at 2025-05-12T11:20:08 (Tb), also detected by Einstein Probe (labelled as EP250512a, Zhao et al. GCN 40437
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 ).

The following trigger information was received after data download through the X-band ground station.

The burst was detected onboard both by the Count-Rate Trigger (CRT) and the Image Trigger (IMT), which produced a sequence of 16 alerts. CRT provided the alert with the best signal-to-noise-ratio (SNR) of 25.26 in the 8-120 keV energy band over a time window of 20.40 seconds starting at Tb. 

The localization of the best alert is R.A., Dec. 215.22, -10.1086 degrees with a 90% confidence level (C.L.) radius of 3.65 arcmin (including systematic error of 2 arcmin added in quadrature).

The ECLAIRs light curve in the 4-120 keV energy band shows a single broad peak structure with a T90 duration of 23.76 (-2.50/+0.48) seconds.

SVOM slewed to the burst.

MXT began observing the field at 2025-05-12T11:22:58.718 UTC, 160 seconds after Tb.

Using the X-band data we found an uncatalogued X-ray source located at R.A., Dec. 215.200, -10.1128 degrees:
R.A. (J2000) = 14h20m48.1s
Dec. (J2000) = -10d06m46.16s
with a 90% C.L. radius of 40 arcseconds including 35 arcseconds added in quadrature.
This location is 1.2 arcminutes from the ECLAIRs onboard position and 50 arcseconds from the EP/FXT one.
The MXT source is rapidly fading and is not detected in the second orbit starting 1.5 hours after slew.

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. ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. MXT was developed jointly by CNES, CEA-IRFU, CNRS-IJCLab, University of Leicester, MPE.

The Burst Advocate (BA) on shift for this burst is Pierre Maggi: pierre.maggi@astro.unistra.fr.
Please contact the BA by email if you require additional information regarding the SVOM follow-up of this burst.