GCN Circular 39550

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

We observed the field of EP250302a detected by Einstein Probe (Trigger ID: 01709132186), using the HMT-0.5m telescope located at Nanshan, Xinjiang, China. Observations started at 15:47:09 on 2025-03-02, i.e., 11.08 min after the EP trigger, and a series of unfiltered frames with different exposures were obtained.

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

R.A.(J2000) = 11:18:03.58
Dec.(J2000) = +33:35:09.06

with an uncertainty of ~0.5 arcsec. The source has R ~ 18.3 mag at a median time of 11.58 min after the EP trigger, calibrated with Legacy Survey DR10 and not corrected for Galactic extinction. We conclude that the source is the optical counterpart of EP250302a.

Observations are still ongoing, and spectroscopy is encouraged.