TITLE:   GCN CIRCULAR
NUMBER:  40402
SUBJECT: EP250508a: SVOM/VT optical candidate
DATE:    25/05/09 05:40:30 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, T. Zhao, R. D. Liang, H. Q. Cheng, W. D. Zhang (NAOC, CAS), Y. D. Hu (GXU), L. Zhang (IHEP), X. L. Chen(YNU) report on behalf of the SVOM and EP Teams:  

SVOM/VT observed the X-ray transient EP250508a (Liang et al., GCN 40390，Zhao et al., GCN 40399) in ToO mode in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.  The observation started at 2025-05-08T11:38:19 UT, about 5.6 hours after the trigger. 

An uncatalogued faint candidate  is detected in both channels within FXT error circle, compared to Legacy Survey, with the coordinate:

 RA= 11:53:01.05 (178.25438 deg) 
 DEC=-19:55:24.2 (-19.92339 deg)
 Error = 0.5 arcseconds
 J2000

The brightness of the candidate is derived as below :

Mid-time              band      mag(AB)         exposure
6.74 hour            VT_R     23.0+/-0.2       71*70 sec
6.75 hour            VT_B     23.6+/-0.3       71*70 sec

Our 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 to confirm the nature of the transient. 

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.