TITLE: GCN CIRCULAR NUMBER: 41242 SUBJECT: GRB 250805A: SVOM/VT optical counterpart DATE: 25/08/06 05:11:56 GMT FROM: Huali Li at at NAOC, SVOM lhl@nao.cas.cn

H. L. Li, Y. N. Ma, L. P. Xin, Y. L. Qiu, C. Wu, Z. H. Yao, 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), A. Li (BNU), Y. D. Hu (GXU) report on behalf of the SVOM/VT team.

SVOM performed a Target of Opportunity observation of GRB 250805A detected by Swift/BAT (Ambrosi et al., GCN 41233; Evans et al., GCN 41234; Page et al., GCN 41240). SVOM/VT began observing the field at 2025-08-05 07:00:42 UTC, 50.4 min after the Swift trigger time, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.

An uncatalogued source, compared to Lagacy survey, is found using VT X-band data, with a distance of 2.7 arcsec to Swift/XRT enhanced position (Beardmore et al., GCN 41238). The optical source is at R.A., Dec 24.216108, -81.402177 degrees:   RA (J2000) = 01:36:51.86 Dec (J2000) = -81:24:07.84 with an uncertainty of 0.5 arcsec.   The source is detected in both VT_B and VT_R images. It decayed by 0.7 mag within 1.89 hours of VT observation. The magnitudes are derived as below:   Mid time | exposure time (s) | band | mag (AB) | mag err --------------------|-------------------|------|----------|-------- 1.24 hours | 37*70 | VT_B | 22.79 | 0.10 1.24 hours | 37*70 | VT_R | 21.16 | 0.04

Our photometry was not corrected for Galactic extinction.

Given the fading of its brightness, the uncatalogued source is proposed to be the optical afterglow of the burst. Our result is consistent with the report by ESO/VLT (Izzo et al., GCN 41241).

The color of VT_B-VT_R~1.5 mag suggests that it shall be a low or intermediate redshift GRB.

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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