TITLE:   GCN CIRCULAR
NUMBER:  40536
SUBJECT: GRB 250521B: SVOM/VT optical upper limit
DATE:    25/05/23 12:43:46 GMT
FROM:    Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>

H. L. Li, Y. L. Qiu, L. P. Xin, 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. Foisseau (APC) and C. Lachaud (APC) report on behalf of the SVOM/VT team.

SVOM/VT conducted ToO follow-up observations of the GRB 250521B (Foisseau et al., GCN 40513). The observation started on 2025-05-21T20:11:45 UT (i.e. 15.87 hour after trigger time) in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channel simultaneously. 

No uncatalogued source was detected, compared to the Legacy survey, within errorboxes of the candidates reported by Swift-XRT (Evans et al., GCN 40517) and EP-FXT (Shui et al., GCN 40532),  down to 3 sigma upper limit of VT_R~24.0 mag (AB) in 109*100 sec stacked images at the mid time of 19.29 hour.

The upper limit is consistent with the report by Angulo et al. (GCN 40521).

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.