TITLE: GCN CIRCULAR NUMBER: 42223 SUBJECT: GRB 251013C: SVOM/VT optical observation DATE: 25/10/13 18:18:36 GMT FROM: SVOM_group svomgroup@bao.ac.cn
J. T. Palmerio (CEA/Irfu), Ny Avo Rakotondrainibe, C. Adami (LAM), H. L. Li, L. P. Xin, Y. L. Qiu, C. Wu (NAOC) report on behalf of the SVOM/VT team:
After the trigger by SVOM/ECLAIRs at 2025-10-13T17:39:42 UTC (T0), SVOM performed an automatic slew on the burst location (Rakotondrainibe et al., GCN 42222). SVOM/VT began observing the field at 2025-10-13T17:44:04, 261.08 seconds after T0, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
From a preliminary analysis of the 1-bit subimage and source list downloaded via VHF network, at least one credible candidate is identified, the details of which are presented below.
VT_ID 117: This bright candidate was flagged as a catalogued source but significantly brighter than its catalogued magnitude. The position of this candidate is R.A., Dec. 345.8356, -0.2103 degrees, corresponding to: R.A. (J2000) = 23h03m20.6s Dec. (J2000) = -0d12m37.1s with an uncertainty of 0.5 arcsec.
This location is within the R90 uncertainty region of the SVOM/MXT onboard localization. The source was detected in both VT_R and VT_B. The candidate's magnitudes are:
| date-obs (UTC) | mid-time | exposure | band | mag(AB) | | -------------------- | ----------- | --------- | ----- | ------------- | | 2025-10-13T17:44:04 | 6.85 min | 6*50 sec | VT_B | 15.38 ± 0.01 | | 2025-10-13T17:44:04 | 6.85 min | 6*50 sec | VT_R | 14.76 ± 0.01 |
Magnitudes were not corrected for dust extinction. The VT color of the counterpart suggests it is not very high redshift or highly extinguished.
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/VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.
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