GCN Circular 38318

B.-T. Wang (YNAO, CAS), T. C. Zheng (PMO, CAS), W. J. Zhang (NAO, CAS), J. Yang (NJU), H. W. Pan (NAO, CAS) on behalf of the Einstein Probe team
 
We report on the detection of an X-ray transient detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, EP241125a (ID: 11900017164). The transient was first detected with WXT at 2024-11-25 00:06:06 (UTC) and lasted for over 150 seconds. The WXT position of EP241125a is R.A.= 48.561 deg, DEC = 37.677 deg (J2000) with an uncertainty of 2.65 arcmin (radius, 90% C.L. statistical and systematic). It has a peak flux of ~8 x 10^-10 erg/s/cm^2 in the 0.5-4 keV band. The average 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 1.90 (+1.12/-0.95) (with a column density fixed at the Galactic value of 1.2 x 10^21 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 2.79 (+1.11/-0.86) x 10^(-10) 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 source position.
 
We have performed a target of opportunity observation with Swift. The Swift XRT observation began at 2024-11-25 16:16:24 with an exposure time of 3500 seconds in the Photon Counting mode. No X-ray source with a signal-to-noise (S/N) ratio greater than 3 is detected within the positional error circle (2.65 arcmin radius) of the EP source. Further multi-wavelength follow-up observations are encouraged to identify the nature of this X-ray transient. 

We thank the Neil Gehrels Swift Observatory team for making the X-ray observation possible. 

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).