TITLE: GCN CIRCULAR NUMBER: 40685 SUBJECT: GRB 250610B: SVOM/VT optical shallow decay DATE: 25/06/11 09:54:28 GMT FROM: Huali Li at at NAOC, SVOM <lhl@nao.cas.cn> Y. L. Qiu, H. L. Li, L. P. Xin, C. Wu, Z. H. Yao, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, Y. N. Ma, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA), A. Saccardi, J. Rodriguez, N. Dagoneau (CEA), C. Van Hove (IJCLab) report on behalf of the SVOM mission team. SVOM/VT made a second ToO observation for GRB250610B (Saccardi et al., GCN 40671; Wang et al., GCN 40682; Wang et al, GCN 40683). The observation was performed in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. With more data, the brightness of the counterpart (Schneider et al., GCN 40676; Gompertz et al., GCN 40677; Li et al., GCN 40678; Bochenek et al., GCN 40684) was fading by about 0.6+/-0.1 mag in both band within 9 hours. The latest brightness is derived as follows: Mid-time after the burst | exposure time| band | mag (AB) | mag err -------------------------|--------------|------|----------|-------- 9.53 hour | 14*70 sec | VT_B | 21.3 ｜ 0.1 9.50 hour | 12*70 sec | VT_R | 21.1 ｜ 0.1 Our photometry was not corrected for Galactic extinction. We noticed that there is a faint blue source at the position of the counterpart in Legency Survey DR10 with g=23.0 mag and r=22.75 mag. Considering the possible host galaxy's brightness and blue color，the photometry at later phase might be contaminated by the host galaxy's light, particularly in the VT_B band. With the slow decaying behavior of X-ray emission (Evans et al., GCN 40679; Liang et al., GCN 40681) and optical at early phase, it is not an ordinary GRB event. More multiband follow-up and spectrum observations are encouraged. 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/40685. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...