TITLE: GCN CIRCULAR NUMBER: 41326 SUBJECT: GRB 250812A: SVOM/VT optical afterglow confirmation DATE: 25/08/12 13:41:54 GMT FROM: Liping Xin at NAOC, SVOM xlp@nao.cas.cn

L. P. Xin, H. L. Li, Y. L. Qiu, C. Wu, Y. N. Ma, Z. H. Yao, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA), Z. M. Wang (BNU) report on behalf of the SVOM/VT team.   SVOM/VT performed a Target of Opportunity observation of GRB 250812A detected by SVOM/Eclairs (Xin et al., GCN 41322). The observation began at 2025-08-12T09:19:03 UTC, 6.55 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.

The optical counterpart (He et al., GCN 41324) within the errorbox of Swift/XRT (Sbarrato et al., GCN 41325) was detected in both VT_R and VT_B. The magnitudes are: Mid-time (h) | exposure time (s) | band | mag (AB) | mag err ---------------|-------------------|------|----------|-------------------------------- 8.29 | 22*70 | VT_B | 19.36 | 0.02 8.29 | 22*70 | VT_R | 18.92 | 0.02

We also noticed that this source was brightening during our observations for about 0.4 magnitudes from 6.67 hours to 8.33 hours after the burst.

Our photometry was not corrected for Galactic extinction.

More VT follow-ups will be scheduled.

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.

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