TITLE: GCN CIRCULAR NUMBER: 41312 SUBJECT: GRB 250807B: SVOM/VT optical observation DATE: 25/08/10 08:51:43 GMT FROM: Yinuo Ma mayn@bao.ac.cn

Y. N. Ma, Z. H. Yao, L. P. Xin, Y. L. Qiu, C. Wu, H. L. Li, X. H. Han, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team.

SVOM/VT performed a Target of Opportunity observation of GRB 250807B detected by Swift/BAT (Klingler et al., GCN 41268). The observation began at 2025-08-09T19:37:17 UTC, 51.903 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.

The optical counterpart (Klingler et al., GCN 41268; Goad et al., GCN 41272; van Dalen et al., GCN 41278; Giudice et al., GCN 41289; Freeberg et al., GCN 41293; Shilling et al,. GCN 41294) was detected in both VT_R and VT_B. The magnitudes are: Mid-time (h) | exposure time (s) | band | mag (AB) | mag err ---------------|-------------------|------|----------|-------- 53.83 | 24*70 | VT_B | 23.7 | 0.3 53.84 | 24*70 | VT_R | 22.9 | 0.3

Our photometry was not corrected for Galactic extinction.

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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