TITLE: GCN CIRCULAR NUMBER: 40530 SUBJECT: GRB 250521D: SVOM/VT optical observations DATE: 25/05/23 00:26:24 GMT FROM: Liping Xin at NAOC, SVOM <xlp@nao.cas.cn> W. Zheng (UCB), L. P. Xin, Y. L. Qiu, H. L. Li, C. Wu, Z. H. Yao, Y. N. Ma, X. H. Han, J. Wang (NAOC), Y. F. Liang (PMO), Z. Q. Wang (GXU) report on behalf of the SVOM/VT team: SVOM performed a Target of Opportunity observation of GRB 250521D detected by Fermi/GBM (Fermi GBM team, GCN 40516). SVOM/VT began observing the position of the counterpart candidate reported by LAST (Konno et al., GCN 40520) at 2025-05-22T05:58:37 UTC, 7.61 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. We clearly detected the candidate in VT both channels. The brightness in AB magnitude was estimated to be: Mid time (hour) | Band | Exposure Time (second) | Magnitude | Magnitude error 7.819 | VT_B | 15*100 | 19.66 | 0.02 7.819 | VT_R | 15*100 | 19.45 | 0.02 However, considering that there is no significant fading in brightness, and also this candidate was detected by GOTO (O'Neill et al., GCN 40522) before the GBM trigger, we also suggest that this candidate is an unrelated object. 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/40530. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...