TITLE: GCN CIRCULAR NUMBER: 39583 SUBJECT: EP250304a: TRT optical counterpart detection DATE: 25/03/04 08:17:49 GMT FROM: Dong Xu at NAOC/CAS dxu@nao.cas.cn

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

We observed the field of EP250304a (Chen et al, GCN 39580), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Cerro Tololo Inter-American Observatory, Chile. Observations started at 02:18:49 UTC on 2025-03-04, i.e., ~1.12 hr after the EP/WXT trigger. A series of B-/V-/R-/I- band frames were obtained.

An uncatalogued and varying optical source is detected within the EP/FXT error circle (Chen et al, GCN 39580) at coordinates

R.A. (J2000) = 13:53:34.68 Dec. (J2000) = -42:48:16.71

with an uncertainty of ~ 1 arcsec.

Preliminary photometry shows that the source has B = 20.5 +/- 0.1 mag at 1.39 hr post-trigger, and R = 20.8 +/- 0.1 mag at 2.4 hr post-trigger, calibrated with SkyMapper DR2 catalog converted using Lupton (2005) equations and not corrected for Galactic extinction.

We thus think this source is the optical counterpart of EP250304a.

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