GCN 45268: GRB 260618B: SVOM/VT continued optical observations indicated a steeper and redder decay

H. L. Li, Y. N. Ma, L. P. Xin, Y. L. Qiu, C. Wu, 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 further observations of GRB 260618B detected by Fermi/GBM (Fermi GBM team et al., GCN 44979; Veres et al., GCN 45010), Fermi/LAT (Gupta et al., GCN 44983), and Konus-Wind (Ridnaia et al., GCN 45038). The observation started at 2026-07-29T02:00:37 UTC, about 40.38 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, GCN 45151; Shi et al., GCN 45014; Ma et al., GCN 45173) showed a steeper decay with a slope of -0.8 in both channels starting from about 21.6 d post trigger, and its color became redder. The following measurements are in AB magnitudes and are not corrected for Galactic extinction:

Mid time (d) | Exposure Time (s) | Band | Brightness (mag)
40.43        | 60*70             | VT_B | 22.9 +/- 0.2
40.43        | 58*70             | VT_R | 22.2 +/- 0.2

The nature of the source remains unclear. More follow-up 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.