TITLE: GCN CIRCULAR NUMBER: 43203 SUBJECT: GRB 251222A / EP251222b: SVOM/VT optical rebrightening DATE: 25/12/23 05:39:41 GMT FROM: Huali Li at at NAOC, SVOM <lhl@nao.cas.cn> H. L. Li, L. P. Xin, Y. L. Qiu, C. Wu, Z. H. Yao, Y. N. Ma, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. R. Xu, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) , H. Yang, S. Guillot (IRAP) report on behalf of the SVOM/VT team. SVOM/VT performed an automatic slew on the burst GRB 251222A (SVOM trigger reference:sb25061207) triggered by SVOM/ECLAIRs (Zhang et al., GCN 40697) and also detected by SVOM/GRM (Wang et al., GCN 43202), and Fermi/GBM (Fermi GBM team, GCN 43186), which is likely associated with EP251202b (Guo et al., GCN 43201). The observation began at 2025-12-22T17:09:44 UTC, i.e. 238 seconds after T0, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. With the data received, the optical counterpart (Palmerio et al., GCN 43819; An et al., GCN 43194; Durbak et al., GCN 43197) was rising at early time to the peak of VT_R~17.37+/-0.02 mag at 389 sec and then fading in both channels with the slope of -0.92. At around 10000 seconds, the brightness was VT_R ~ 18.09+/-0.02 mag confirming a rebrightening between the epoch, and then decayed with the slope of -0.84. The result would be be refined with more data received. The measurements are derived in AB magnitude as follows: mid-time | exposure time | band | mag (AB) ------------|----------------- |------------------- 263 sec | 50 sec | VT_B | 18.27+/-0.03 263 sec | 50 sec | VT_R | 17.57+/-0.02 5.242 hour | 100 sec | VT_B | 19.28+/-0.04 5.242 hour | 100 sec | VT_R | 18.56+/-0.03 Our photometry was not corrected for Galactic extinction. More VT follow ups are scheduled. The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Centre 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/43203. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...