GCN Circular 40277

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

We report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250428b (GCN Notice ID 01799135422). The WXT position of the source is R.A. = 220.479 deg, DEC = 2.09 deg (J2000) with an uncertainty of 2.7 arcmin in radius (90% C.L. statistical and systematic). A preliminary analysis of the EP-WXT data shows that the transient began at around 2025-04-28T06:05:31(UTC) and lasted for a few hundreds of seconds, with a peak flux of around 3 x 10^-10 erg/cm^2/s. The averaged WXT spectrum can be fitted by an absorbed power law model with a Galactic hydrogen column density of 3.77 x 10^20 cm^-2 and a photon index of 1.9 (+0.2, -0.2). The averaged unabsorbed 0.5-4 keV flux is 3.3 (+2.2, -1.3) x 10^-11 erg/cm^2/s. The uncertainties are at the 90% confidence level for the above parameters.

A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed from 2025-04-29T13:41:27 (UTC). Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 220.4586 deg, DEC = 2.1002 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic). Further information will be updated when the telemetry data is received.

The contact TA of EP250428b is Q. Y. Wu. Please contact him via email qywu@bao.ac.cn if needed.

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).