TITLE: GCN CIRCULAR NUMBER: 40285 SUBJECT: EP250428b: refined analysis of the EP-FXT follow-up observation DATE: 25/04/30 09:16:55 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

Q. Y. Wu, M. J. Liu, T. Zhao, Y. J. Song (NAO, CAS), H. Z. Wu (HUST), C. C. Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team:

After the EP-WXT detection of the fast X-ray transient EP250428b (Liu et al., GCN 40277), a follow-up observation was performed by EP-FXT at 2025-04-29T14:04:00 (UTC), about 32 hours after the detection, with an exposure time of 3.0 ks. On-ground analysis of the EP-FXT data identified an uncatalogued X-ray source within the EP-WXT error circle, at the coordinates (J2000): R.A., Dec. = 220.4575, 2.0989 deg, with an uncertainty of 10 arcsec (radius, 90% confidence level, including both statistical and systematic errors). The unabsorbed 0.5-10 keV flux is estimated to be 1.2 (+0.4, -0.4) x 10^-13 erg/cm^2/s, derived using an absorbed power law model with the hydrogen column density fixed to the Galactic value of 3.77 x 10^20 cm^-2 and the photon index fixed at 2.0, due to the limited photon statistics. All uncertainties quoted above are at the 90% confidence level.

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

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