TITLE:   GCN CIRCULAR
NUMBER:  42453
SUBJECT: GRB 251025B: SVOM/VT optical bump
DATE:    25/10/26 04:30:16 GMT
FROM:    Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>

H.L. Li, L.P. Xin, Y.L. Qiu, C. Wu, Y.N. Ma, Z.H. Yao, X.H. Han, J. Wang, H.B. Cai, W.J. Xie, Y. Xu, J.R. Xu, P.P. Zhang, Y.J. Xiao, J.S. Deng, J.Y. Wei (NAOC), J. Palmerio (CEA), J.X. Cao, X. Tian (GXU), S. Hussein (IJCLab), M. Brunet (IRAP) report on behalf of the SVOM team.

SVOM performed an automatic slew on the burst GRB 251025B triggered by SVOM/ECLAIRs (Julakanti et al., GCN 42437) in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. The observation began at  2025-10-25T14:26:52, i.e., 167 sec post trigger. 
 
The optical counterpart (Wu et al., GCN 42438, 42440; Budnev et al., GCN 42439; Mohan et al., GCN 42447; Fung et al., GCN 42450; Pereyra et al., GCN 42452) consistent with the location of Swift-XRT (Beardmore et al., GCN 42445) was clearly detected in both VT channels. The brightness was decaying with a slope of -0.7 before 3000 sec post trigger, followed by a bump with the peak of VT_R~19.59+/-0.05 mag and VT_B~19.95+/-0.05 mag at the mid time of 5.24 hours post trigger. 

Our photometry was estimated in AB magnitude, and not corrected for Galactic extinction.

More follow-up for this burst is encouraged.

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.