TITLE:   GCN CIRCULAR
NUMBER:  40159
SUBJECT: EP250225a: SVOM/VT optical brightening
DATE:    25/04/17 07:50:33 GMT
FROM:    SVOM_group <svomgroup@bao.ac.cn>

X. H. Han(NAOC), L. Zhang(IHEP), X. L. Chen(YNU), Y. D. Hu(GXU), L. P. Xin, Y. L. Qiu, H. L. Li, C. Wu, Z. H. Yao, Y. Xu, P. P. Zhang, W. J. Xie, J. Wang, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC) report on behalf of the SVOM mission team.

SVOM/VT conducted ToO follow-up observations of the fast X-ray transient EP250225a (Jiang et al., GCN 39475). The observation started on 2025 Feb 26 09:37:03 UT in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channel simultaneously. 

The candidate (Malesani et al., GCN 39516; Malesani et al., GCN 39573) was clearly detected in stacked images of both channels.

The brightness in AB magnitude was estimated to be:

Mid time (hour)  | Band | Exposure Time (second) | Magnitude | Magnitude error
21.80            | VT_B |          3000          |   22.31   |      0.13
21.80            | VT_R |          3000          |   21.50   |      0.10

The source was brightening with about 0.4 mag within 3.3 hours in both channels during our observations. This result is consistent with the report of the brightening compared with the archival brightness in the Legacy survey (Malesani et al., GCN 39516).


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.