TITLE: GCN CIRCULAR NUMBER: 40500 SUBJECT: GRB 250520A：SVOM/VT optical candidate DATE: 25/05/20 10:45:18 GMT FROM: Liping Xin at NAOC, SVOM xlp@nao.cas.cn
L. P. Xin, Y. L. Qiu, H. L. Li, C. Wu, Z. H. Yao, Y. N. Ma, X. H. Han, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC) report on behalf of the SVOM/VT team:
SVOM performed a Target of Opportunity observation of GRB 250520A detected by Swift/BAT (Eyles-Ferris et al., GCN 40491), SVOM/GRM (Wang et al., GCN 40495) and AstroSat(Tembhurnikar et al., GCN 40497). SVOM/VT began observing the field at 2025-05-20T03:51:16 UTC, 1.15 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
An uncatalogued faint source, compared to PanStarrs catalogue, is found using VT X-band data, within the error box of Swift/XRT (Goad et al., GCN 40494) at RA=282.28578, Dec=-11.86939 degrees:
R.A.(J2000) = 18:49:08.58 Dec.(J2000) = -11:52:09.79 with an uncertainty of 0.5 arcsec.
The source is detected in VT_R only. The magnitude is estimated to be 23.2+/-0.1 mag with an effective exposure time of 37*70 seconds at the mid time of 2.14 hours after the burst.
The photometry was estimated in AB magnitude and not corrected for Galatic extinction.
Give the faintness of the candidate, we cannot determine whether it is fading. More follow-ups are encouraged.
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.
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