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GCN Circular 41758

Subject
EP250908b: SVOM/VT optical candidate
Event
EP250908b
Date
2025-09-09T03:20:04Z (4 days ago)
From
Yinuo Ma <mayn@bao.ac.cn>
Via
Web form
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 EP250908b detected by EP/WXT (Yin et al., GCN 41752Loading...
 
 ). SVOM/VT began observing the field at 2025-09-08T19:29:58 UTC, 1.061 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. 

With X-band data availible, an uncatalogued fading optical candidate within the error box of EP/FXT, compared to Legacy survey, was clearly detected in both VT_B and VT_R. The magnitudes are:

mid time (h) | exposure time (s) | band | mag (AB) | mag err
-------------|-------------------|------|----------|--------
    1.491    |      57*50        | VT_B |  21.13   |  0.06 
    3.103    |      32*50        | VT_B |  22.5    |  0.2 
    1.484    |      60*50        | VT_R |  20.66   |  0.06 
    3.103    |      32*50        | VT_R |  21.4    |  0.1    

Our photometry was not corrected for Galactic extinction.

We proposed that this is the optical candidate for the trigger. 

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.