TITLE: GCN CIRCULAR NUMBER: 40052 SUBJECT: EP250404a: Nanshan/HMT optical counterpart detection DATE: 25/04/04 15:12:14 GMT FROM: Dong Xu at NAOC/CAS dxu@nao.cas.cn

S.Q. Jiang, Z.P. Zhu, J. An, X. Liu, D. Xu (NAOC), S.Y. Fu (HUST), X. Gao (Urumqi No.1 Senior High School), J.Z. Liu (XAO) report:

We observed the field of EP250404a detected by EP (Hu et al., GCN 40051), which is temporally and spatially consistent with GRB 250404A detected by Fermi/GBM (GBM team, GCN 40050), using the HMT-0.5m telescope located at Nanshan, Xinjiang, China. Observations started at 14:24:59 UT on 2025-04-04, i.e., 300 s after the EP/WXT trigger time, and a series of unfiltered frames with different exposures were obtained.

An uncatalogued and varying optical source is detected within the EP/FXT error circle at coordinates:

R.A.(J2000) = 08:20:14.54 Dec.(J2000) = +35:31:41.57

with an uncertainty of ~0.5 arcsec. The source has R ~ 16.1 mag in the first image, calibrated with the nearby Pan-STARRS field and not corrected for Galactic extinction.

Observations are still ongoing.

View this GCN Circular online at https://gcn.nasa.gov/circulars/40052. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...