TITLE: GCN CIRCULAR NUMBER: 45451 SUBJECT: GRB 2600818B: SVOM/VT optical upper limit and continuous observations DATE: 26/08/27 06:21:49 GMT FROM: Huali Li at at NAOC, SVOM <lhl@nao.cas.cn> H. L. Li, L. P. Xin, C. Wu, Y. L. Qiu, Y. N. Ma, Z. H. Yao, J. R. Xu, J. Wang, X. H. Han, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC), J. T. Palmerio, Bertrand Cordier (CEA/Irfu) report on behalf of the SVOM/VT team. SVOM/VT performed several ToO observations for GRB 260818B, which was triggered by SVOM/ECLAIRs (sb26081807, Cordier et al., GCN 45398; Yang et al., GCN 45405). This burst was also detected by SVOM/GRM (Ren et al., GCN 45411), HXMT (Wang et al., GCN 45408). The earliest observation started at 2026-08-18T23:33:18 UTC, about 3.4 hr post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously. Additional ToO observations were performed at 1.02 d and 7.5 d post trigger. No credible candidate was found in the stacked images for each epoch, within the SVOM/ECLAIRs error box (Cordier et al., GCN 45398), the VLA radio sources (Schroeder et al., GCN 45418) and the Einstein Probe X-ray sources (Wu et al., GCN 45407). The typical 5-sigma upper limits are 23.0 mag in both channels. Our measurements are in AB magnitudes and are not corrected for Galactic extinction. This non-detection is consitent with the report from LCO (Shi et al., GCN 45399), SVOM/COLIBRÍ (Guelfand et al., GCN 45401), NOT (O'Neill et al., GCN 45403) and SVOM-CGFT (Guan et al., GCN 45417). We noticed that there seemed to be an optical brightening trend for the VLA radio source S10 (Schroeder et al., GCN 45418). Its brightness in VT_R was 22.2+/-0.3 mag at 5.1 hr to 21.7+/-0.2 mag at day 7. The position of S10 is spatially coincident with EPF_J025955.0+072535 reported by EP-FXT (Wu et al., GCN 45407). In the vicinity of S10, an optical candidate was proposed by SVOM/COLIBRÍ (FM-GFT) (Ducoin et al., GCN 45450). Some contributions from the COLIBRÍ candidate may be blended into our photometry, since the angular separation between the two sources is only 1.7 arcsec. Given the limited spatial resolution of VT and the faintness of the candidate (Ducoin et al., GCN 45450), we cannot confirm its detection. But the light curve of S10 shows some positive trend which needs to be further confirmed. 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/45451. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...