GCN 45320: GRB 260808B: SVOM/VT optical afterglow confirmation

H. L. Li, C. Wu, L. P. Xin, Y. L. Qiu, 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), J. T. Palmerio (CEA/Irfu), M. Dinatolo (U. Milano) report on behalf of the SVOM/VT team.

SVOM/VT performed observations with automatic slew for GRB 260808B triggered by SVOM/ECLAIRs (sb26080812, Bouchet et al., GCN 45316) and detected by SVOM/MXT (Bouchet et al., GCN 45316). The observation started at 2026-08-08T14:24:26 UTC, i.e., 208 seconds post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.

The optical candidate (Turpin et al., GCN 45318) was detected in both channels. The position is at R.A., Dec. = 345.233538, 8.478528 degrees, equivalent to:
   R.A. (J2000) = 23:00:56.05 
   Dec. (J2000) = +8:28:42.70
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
 233 s     VT_B       50 s        20.9 +/- 0.2 mag
 233 s     VT_R       50 s        20.3 +/- 0.1 mag
1.51 h     VT_B    16*50 s        22.8 +/- 0.2 mag
1.51 h     VT_R    16*50 s        22.0 +/- 0.2 mag

The source faded rapidly with a temporal decay slope of -1.9 in both channels during our early observations taken within 400 seconds post trigger.

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.