TITLE: GCN CIRCULAR NUMBER: 39804 SUBJECT: EP250321a: TRT optical counterpart detection DATE: 25/03/21 08:39:06 GMT FROM: sqjiang at NAOC sqjiang@bao.ac.cn

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

We observed the field of EP250321a detected by EP/WXT (Hu et al., GCN 39800) using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Sierra Remote Observatories in California. We obtained several 300 s frames in the R and I filters.

An uncatalogued and decaying optical source is detected within the error circle at coordinates

R.A. (J2000) = 11:57:03.02 Dec. (J2000) = +17:21:45.94

with an uncertainty of ~0.5 arcsec. The source has R ~ 19.7 mag at 1.24 hr and I ~ 18.8 mag at 1.72 hr after the EP trigger, calibrated with nearby Pan-STARRS DR2 stars and not corrected for Galactic extinction.

We think that the source is very likely the optical counterpart of EP250321a.

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