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.