TITLE: GCN CIRCULAR NUMBER: 41340 SUBJECT: EP250812a: SVOM/VT optical counterpart DATE: 25/08/13 09:40:18 GMT FROM: Liping Xin at NAOC, SVOM xlp@nao.cas.cn

L. P. Xin, H. L. Li, Y. N. Ma, Y. L. Qiu, C. Wu, Z. H. Yao, 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 two Target of Opportunity observations of EP250812a detected by EP/WXT (Zhang et al., GCN 41327). The first epoch is from 2025-08-12T20:32:53 UTC, 7.38 hours after the EP trigger time. The second epoch is from 2025-08-13T01:47:59, 12.64 hours post the trigger.

No any uncatalogued sources, compared to Lagecy survey, was found using VT X-band data, within the errorbox of EP/FXT (Zhang et al., GCN 41327, GCN 41339) or Swift/XRT (Evans et al., GCN 41330).

However, there is a cataloged source in Legacy survey within the errorbox of Swift/XRT source1 with a distance of 3.4 arcseconds at R.A.=30.993037 Dec=-9.318883 degrees: RA (J2000) = 02:03:58.329 Dec (J2000) = -09:19:07.98 with an uncertainty of 0.5 arcsec.

The cataloged source was detected in both VT_B and VT_R images. The magnitudes are derived as below:   DeltaT(Mid time) | exposure time (s) | band | mag (AB) | mag err ---------------------- |-------------------|------|----------|-------- 7.62 hours | 26*70 | VT_B | 22.52 | 0.15 7.62 hours | 26*70 | VT_R | 22.04 | 0.12 14.34 hours | 75*70 | VT_B | 23.10 | 0.16 14.34 hours | 67*70 | VT_R | 22.74 | 0.15

Our photometry is not corrected for Galactic extinction.

Given the fading of the source in our observations, we proposed that it is the counterpart 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.

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