TITLE: GCN CIRCULAR NUMBER: 42061 SUBJECT: GRB 251002A: SVOM/VT optical observation DATE: 25/10/02 20:58:44 GMT FROM: SVOM_group svomgroup@bao.ac.cn

J. T. Palmerio (CEA/Irfu), H. L. Li, L. P. Xin, Y. L. Qiu, C. Wu report on behalf of the SVOM/VT team:

After the trigger by SVOM/ECLAIRs at 2025-10-02T20:14:54 UTC (T0), SVOM performed an automatic slew on the burst location (Saccardi et al., GCN 42060). SVOM/VT began observing the field at 2025-10-02T20:19:44, 289.75 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 36: This candidate was flagged as un uncatalogued source. The position of this candidate is R.A., Dec. 13.9082, -5.5671 degrees, corresponding to: R.A. (J2000) = 0h55m38.0s Dec. (J2000) = -5d34m01.5s with an uncertainty of 1 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)  | | -------------------- | ----------- | --------- | ----- | ------------- | | 2025-10-02T20:19:44  | 6.50 min  | 4*50 sec  | VT_B  | 16.81 ± 0.01  | | 2025-10-02T20:19:44  | 6.50 min  | 4*50 sec  | VT_R  | 16.24 ± 0.01  | Magnitudes were not corrected for dust extinction.

Given the VT color of the counterpart, it might be a low redshift gamma-ray burst.

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.

The SVOM/VT point of contact for this burst is Jesse Palmerio: jesse.palmerio@cea.fr.

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