TITLE:   GCN CIRCULAR
NUMBER:  40443
SUBJECT: GRB 250512B/EP250512a：SVOM/VT bright optical counterpart
DATE:    25/05/12 15:15:31 GMT
FROM:    Liping Xin at NAOC, SVOM <xlp@nao.cas.cn>

H. L. Li, L. P. Xin, Y. L. Qiu, C. Wu,  Z. H. Yao, 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),  P. Maggi (ObAS), Y. N. Ma, D. H. Zhao(NAOC) report on behalf of the SVOM/VT Instrument Team: 

GRB 250512B (sb25051203), triggered both by SVOM/Eclairs (Maggi  et al., GCN 40439) and Einstein Probe(EP/WXT) (labeled as EP250512a, Zhao et al., GCN 40437), was observed by on-board SVOM/VT after the automatic slew of the satellite. The VT conducted observations in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.
 
With the downlinked X-band data, the optical counterpart was clearly detected within the error box of EP/FXT(Zhao et al., GCN 40437)  in both VT_B and VT_R bands. The earliest observation started on 2025-05-12T11:23:21 UTC (i.e. 193 seconds after the SVOM trigger time).

The position of the counterpart is RA=215.20368 deg, DEC= -10.10071 deg, which is equivalent to:
R.A.=14:20:48.88,
DEC.=-10:06:02.56,  
J2000, 
Error=0.5 arcseconds

The light curves show a brightening with a peak of 19.3+/-0.1 mag in VT_R and 21.0+/-0.2 mag in VT_B at 668 seconds after the burst. The photometry is in the AB system and is not corrected for Galactic extinction.
 
Given the color of VT_B-VT_R~1.7 mag, it might be a low or intermediate redshift GRB (Wang et al., 2020).
  
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.