GCN Circular 38861

R.-Z. Li (YNAO, CAS), X.-L. Chen (YNU), K. Chatterjee (YNU), Y.-L. Hua (PMO) and Y. Liu (NAOC) report on behalf of the Einstein Probe team:

We report on the detection of an X-ray transient, designated EP250108a, by the Wide-field X-ray Telescope (WXT, trigger ID: 06800000356) on board the Einstein Probe (EP) mission. The transient event was detected at 2025-01-08T12:30:28.34 (UTC). The WXT position of the source is R.A. = 55.623 deg, DEC = -22.509 deg (J2000) with an uncertainty of 2.2 arcmin in radius (90% C.L. statistical and systematic).

The transient event lasted for more than 2500 seconds. The peak flux in the 0.5-4 keV is around 1.4 x 10^-10 erg/s/cm^2. The average 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 1.35 +/- 0.40 (with a Galactic column density fixed at 1.6 x 10^20 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 4.2 (+1.2/-0.9) x 10^-11 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

A follow-up observation with the Follow-up X-ray Telescope (FXT) on board EP has been scheduled. Further observations are encouraged to explore the origin of the transient.

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). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.