GCN Circular 39625

Y.-H. I. Yin (NJU), Q. Y. Wu (NAO, CAS), J. H. Wu (GZHU), H. W. Pan (NAO, CAS) report on behalf of the Einstein Probe team:
 
We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250308a. The transient triggered EP-WXT (ID: 01709132392) at 2025-03-08T17:03:57 (UTC). The WXT position of the source is R.A. = 210.581 deg, DEC = 18.515 deg (J2000) with an uncertainty of 2.9 arcmin in radius (90% C.L. statistical and systematic). The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 2.37 x 10^20 cm^-2 and a photon index of 2.36 (-0.59/+0.61). The derived average unabsorbed 0.5-4 keV flux is 3.62 (-0.82/+1.18) x 10^(-10) erg/s/cm^2.

The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 165 seconds after the trigger with an exposure time of 5.5 ks. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 210.5875, DEC = 18.4967 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent positionally with the WXT transient. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 2.37 x 10^20 cm^-2 and a photon index of 2.59 (-0.07/+0.08). The derived average unabsorbed 0.5-10 keV flux is 9.71 (-0.42/+0.45) x 10^(-13) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. 

No previously known bright X-ray sources are found within the error circle around the transient position. Further follow-up observations are encouraged to identify the nature of this X-ray 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).