TITLE:   GCN CIRCULAR
NUMBER:  45001
SUBJECT: GRB 260618B: SVOM/VT optical candidate
DATE:    26/06/19 13:38:11 GMT
FROM:    Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>

H. L. Li, L. P. Xin, Y. L. Qiu, C. Wu, J. R. Xu, Y. N. Ma, Z. H. Yao, 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), J. T. Palmerio (CEA/Irfu) report on behalf of the SVOM/VT team.

SVOM/VT performed ToO observations for GRB 260618B detected by Fermi-LAT (Gupta et al., GCN 44983) and Fermi-GBM (the Fermi GBM team, GCN 44979).  The observation started at 2026-06-19T01:44:32 UTC, i.e., 8.83 hours post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.

An uncatalogued optical candidate was clearly detected in both channels compared with the Legacy Survey, with an angular separation of 8.7 arcminutes from the Fermi-LAT position (Gupta et al., GCN 44983). The position is at R.A., Dec. = 354.333559, -44.735518 degrees, equivalent to:
   R.A. (J2000) =  23:37:20.05 
   Dec. (J2000) = -44:44:07.87
with an uncertainty of 0.5 arcsec.

The following measurements are in the AB magnitude and are not corrected for Galactic extinction:

Mid time | Band | Exposure Time | Brightness
10.53 h    VT_B    48*70 s       20.8 +/-0.1 mag
10.53 h    VT_R    41*70 s       20.9 +/-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.