TITLE:   GCN CIRCULAR
NUMBER:  45151
SUBJECT: GRB 260618B:  SVOM/VT optical observations indicate shallow decay and reddening
DATE:    26/07/14 02:57:05 GMT
FROM:    Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>

H. L. Li, L. P. Xin, Y. L. Qiu, C. Wu, Y. N. Ma, Z. H. Yao, J. R. Xu, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC) and J. T. Palmerio (CEA/Irfu) report on behalf of the SVOM/VT team.

SVOM/VT performed ToO observation for GRB 260618B detected by Fermi-LAT (Gupta et al., GCN 44983), Fermi-GBM (Fermi GBM team, GCN 44979; Veres et al., GCN 45010) and Konus-Wind (Ridnaia et al., GCN 45038). The latest observation started at 2026-07-10T09:59:57 UTC, about 21.71 d post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.

The optical candidate (Li et al., GCN 45001; Shi et al., GCN 45014) became fainter in both channels and redder relative to earlier observations (Li et al., GCN 45001). The following measurements are in the AB magnitude and are not corrected for Galactic extinction:

Mid time | Band | Exposure Time | Brightness
21.73 d    VT_B    37*70 s       22.3 +/- 0.2 mag
21.73 d    VT_R    34*70 s       21.6 +/- 0.1 mag

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.