TITLE: GCN CIRCULAR NUMBER: 41485 SUBJECT: EP250821a: SVOM/VT optical observation DATE: 25/08/22 03:16:13 GMT FROM: zhyao@bao.ac.cn Z. H. Yao, Y. N. Ma, L. P. Xin, H. L. Li, Y. L. Qiu, C. Wu, 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. Palmerio (CEA) report on behalf of the SVOM/VT team. SVOM performed a Target of Opportunity observation of EP250821a detected by EP/WXT (Hu et al., GCN 41459). SVOM/VT began observing the field at 2025-08-21T11:23:41.5 UTC, 4.087 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. An uncatalogued source is found using VT X-band data, within the error box of EP/FXT (Hu et al., GCN 41459; Liang et al., GCN 41467), Swift/XRT and UVOT (Dichiara et al., GCN 41469) and Las Cumbres (Li et al., GCN 41470) at R.A., Dec 289.88564, -43.38115 degrees: RA (J2000) = 19:19:32.55 Dec (J2000) = -43:22:52.14 with an uncertainty of 0.5 arcsec. The source is detected in both VT_R and VT_B and was fading by ~0.5 mag between the first 2 VT observing sequences, the magnitudes are: T-T0 (h) | Exposure Time (s) | Band | Mag (AB) | Mag err --------------------|-------------------|------|----------|-------- 4.087 | 70 | VT_B | 19.44 | 0.12 4.087 | 70 | VT_R | 18.49 | 0.05 6.363 | 70 | VT_B | 19.93 | 0.05 6.363 | 70 | VT_R | 19.02 | 0.03 Our photometry was not corrected for Galactic extinction. The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Centre 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/41485. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...