TITLE: GCN CIRCULAR NUMBER: 39486 SUBJECT: EP250226a / GRB 250226A: TRT optical counterpart observation DATE: 25/02/26 08:35:09 GMT FROM: Dong Xu at NAOC/CAS dxu@nao.cas.cn

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

We observed the field of EP250226a detected by EP/WXT (Jiang et al., GCN 39482), which is very likely the same event of GRB 250226A detected by Fermi/GBM (Fermi GBM team, GCN 39479), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Cerro Tololo Inter-American Observatory, Chile. We obtained several 300 s frames in the Sloan-r filter.

An uncatalogued and decaying optical source is detected within the EP/FXT error circle (Jiang et al., GCN 39482) at coordinates

R.A. (J2000) = 14:57:03.63 Dec. (J2000) = +20:58:32.31

with an uncertainty of about 1.0 arcsec. The source has r ~ 17.7 mag at 21.8 min after the EP trigger, calibrated with PanSTARRS DR2 stars and not corrected for Galactic extinction.

We think that the source is the optical counterpart of EP250226a / GRB 250226A.

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