TITLE:   GCN CIRCULAR
NUMBER:  45634
SUBJECT: GRB 260918A: SVOM/VT optical upper limits
DATE:    26/09/18 15:27:31 GMT
FROM:    Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>

H. L. Li, Y. N. Ma, C. Wu, L. P. Xin, Y. L. Qiu, Z. H. Yao, J. R. Xu, L. J. Chen, 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), Yun Wang (UCB; PMO) and Yehao Cheng(YNU) report on behalf of the SVOM/VT team.

SVOM/VT performed an automatic slew on the burst GRB 260918A triggered by SVOM/ECLAIRs (sb26091801, Zhao et al., GCN 45619).  The observation started at 2026-09-18T03:53:19 UTC, 323 sec in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
 
No credible candidates were identified in our single or stacked images within the EP-FXT (Zou et al., GCN 45632) and ECLAIRs (Zhao et al., GCN 45619) error box, compared with the Legacy Survey.

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

Mid time | Band | Exposure Time |  5-sigma upper limit
8.7 min    VT_B      8*50  sec       > 22.2 mag    
8.7 min    VT_R      8*50  sec       > 22.1 mag
1.51 hr    VT_B     56*50  sec       > 23.1 mag    
1.51 hr    VT_R     55*50  sec       > 23.0 mag
 
Our non-detection is consistent with the reports from SVOM/COLIBRÍ (Méndez et al., GCN 45620) and LCO (Yin et al., GCN 45622).

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.