TITLE: GCN CIRCULAR NUMBER: 40260 SUBJECT: EP250427a: TRT optical counterpart detection DATE: 25/04/27 10:13:08 GMT FROM: Dong Xu at NAOC/CAS dxu@nao.cas.cn

X. Liu, Z.P. Zhu, S.Q. Jiang (NAOC), S. Tinyanont, R. Anutarawiramkul, P. Butpan (NARIT), S.Y. Fu (HUST), J. An, Z. Fan, W.X. Li, N.C. Sun, Y.N. Wang, D. Xu (NAOC) report on behalf of a large collaboration:

We observed the field of EP250427a (Wang et al., GCN 40257), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Cerro Tololo Inter-American Observatory, Chile. Observations started at 07:24:25.019 UTC on 2025-04-27, i.e., ~3.67 hr after the EP/WXT trigger and 6x200 s frames in the Sloan r-band were obtained.

An uncatalogued and varying optical source is detected within the EP/FXT error circle (Wang et al., GCN 40257) at coordinates

R.A. (J2000) = 18:29:05.73 Dec. (J2000) = +7:33:46.27

with an uncertainty of ~ 0.5 arcsec. Preliminary photometry shows that the source has r ~ 17.9 mag at 3.96 hr post-trigger, calibrated with Pan-STARRS DR2 catalog and not corrected for Galactic extinction.

We think that this source is the optical counterpart of the event.

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