GCN Circular 41735

Q. C. Liu (THU), Y. H. Cheng (SWIFAR,YNU), X. Mao, G. J. Yang, Y. Liu (NAO,CAS) on behalf of the Einstein Probe team:

We report on the detection of a fast X-ray transient, designated EP250905b, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient event started at 2025-09-05T19:18:28 (UTC). The position of the source is R.A. = 90.724 deg, DEC = -54.167 deg (J2000) with an uncertainty of 2.4 arcmin in radius (90% C.L. statistical and systematic).

The transient event lasts for nearly 200 seconds. No automated X-ray follow-up observation was performed due to the solar angle constraint. The average 0.5-4 keV spectrum can be fitted by an absorbed power law with a photon index of 1.4 (+0.6/-0.6). The column density is fixed at the Galactic value of 7.64 x 10^20 cm^-2. The derived average unabsorbed 0.5-4 keV flux is 7 (+4/-2) x 10^-10 erg/s/cm^2. The peak flux is around 2.9 x 10^-9 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. 

Follow-up observations with the Follow-up X-ray Telescope (FXT) on board EP have been scheduled, and further information will be updated when the telemetry data received.

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).