TITLE:   GCN CIRCULAR
NUMBER:  41667
SUBJECT: EP250827b: SVOM/VT observation of AT 2025wkm
DATE:    25/09/03 05:04:52 GMT
FROM:    Yinuo Ma <mayn@bao.ac.cn>

Y. N. Ma, Z. H. Yao, L. P. Xin, Y. L. Qiu, C. Wu, H. L. Li, 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), J. Palmerio (CEA) report on behalf of the SVOM/VT team. 

SVOM performed a Target of Opportunity observation of EP250827b/AT 2025wkm (Schroeder et al., GCN 41635). SVOM/VT began observing the field at 2025-09-02T09:14:40 UTC,  6.12 days after the X-ray detection, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. 

With X-band data availible, the source consistent with AT 2025wkm (Schroeder et al., GCN 41635; Hall et al., GCN 41636; Sevilla & Ho, GCN 41639; Hall et al., GCN 41641; Eyles-Ferris & Starling, GCN 41655) was clearly detected in both VT_B and VT_R. However, the source in VT_R was contaminated by the blooming of bright star, so the magnitude of VT_B is:

mid time (day) | exposure time (s) | band | mag (AB) | mag err
---------------|-------------------|------|----------|--------
      6.24     |      37*70        | VT_B |  19.81   |  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 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.