TITLE: GCN CIRCULAR NUMBER: 41409 SUBJECT: GRB 250818B: SVOM/VT optical observation DATE: 25/08/18 07:13:51 GMT FROM: SVOM_group svomgroup@bao.ac.cn

Z. H. Yao, L. P. Xin, H. L. Li, Y. L. Qiu, C. Wu, Y. N. Ma, 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. Q. Wang (GXU), Y. F. Liang (PMOC) report on behalf of the SVOM/VT team.

SVOM performed an automatic slew on the burst GRB 250818B triggered by SVOM/ECLAIRs (Wang et al., GCN 41405). SVOM/VT start the observations at 2025-08-18T03:32:27.5, 198.5 seconds after the burst, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.

With X-band data availible, the optical counterpart (GOTO25fzq, Kumar et al., GCN 41406) at the position consistent with the locations of Swift/XRT (Ferro et al., GCN 41407), was clearly detected in VT_B. The magnitudes are:

T-T0 (s) | exposure time (s) | band | mag (AB) | mag err ------------------|-------------------|------|----------|-------- 198.5 | 50 | VT_B | 17.61 | 0.03 3963.5 | 50 | VT_B | 19.02 | 0.03

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.

The SVOM point of contact for this burst is Ziqi Wang (ziqi.wang@st.gxu.edu.cn) Please contact the BA by email if you require additional information regarding the SVOM follow-up of this burst.

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