TITLE: GCN CIRCULAR NUMBER: 38904 SUBJECT: EP250109a/GRB 250109A:SVOM/VT optical counterpart DATE: 25/01/11 02:50:01 GMT FROM: Liping Xin at NAOC, SVOM xlp@nao.cas.cn

SVOM/VT commissioning team: Y. L. Qiu, H. L. Li, L. P. Xin, C. Wu, X. H. Han, J. Wang, W. J. Xie, H. B. Cai, Y. Xu, Y. J. Xiao, P. P. Zhang, J. S. Deng, L. Lan, X. M. Lu, R. S. Zhang, Z. H. Yao (NAOC), J. Zhang, L. J. Dan, G. Y. Zou, C. J. Wang, Y. F. Du, C. Huang (XIOPM)

SVOM JSWG: Jian-Yan Wei (NAOC), Bertrand Cordier (CEA), Shuang-Nan Zhang (IHEP), Stéphane Basa (LAM), Arnaud Claret (CEA), Zi-Gao Dai (USTC), Frédéric Daigne (IAP), Jin-Song Deng (NAOC), Olivier Godet (IRAP), Andrea Goldwurm (APC), Diego Götz (CEA), Xu-Hui Han (NAOC), Cyril Lachaud (APC), En-Wei Liang (GXU), Yu-Lei Qiu (NAOC), Susanna Vergani (Obs.Paris), Jing Wang (NAOC), Chao Wu (NAOC), Li-Ping Xin (NAOC), Shao-Lin Xiong (IHEP), Bing Zhang (UNLV)

report on behalf of the SVOM team:

The further analysis of the candidate(Qiu et al., GCN 38872) for the first ToO observation EP250109a / GRB 250109A(Li et al., GCN 38864; Fermi GBM team, GCN 38873, DeLaunay et al., GCN 38900) showed that it was fading for about 0.5 mag from 4.13 hours to 9.65 hours.

The SVOM/VT conducted the second ToO follow-up observations for the transient in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channel simultaneously, from 31.27 hours to 38.79 hours after the burst.

The candidate was still clearly detected in VT_R stacked image with VT_R=23.2+/-0.2 in AB magnitude at the mid time of 35.2 hours post the burst, with a total exposure time of 102*100 seconds.

Given the continuous fading of about 0.9 mag during VT_R total observations, as well as the optical detection of Zeiss-2000 (Moskvitin et al., GCN 38901) and the XRT detection (Kennea et al., GCN 38899), it is confirmed that this source is the optical afterglow of the burst.

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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