GCN Circular 43162

Q. C. Liu (THU), Y. Wang (PMO), H. Y. Liu, H. W. Pan (NAO,CAS) on behalf of the Einstein Probe team:

We report on the detection of a fast X-ray transient, designated EP251220a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient event started at 2025-12-20T08:26:43 (UTC). The position of the source is R.A. =17.322 deg, DEC = 19.080 deg (J2000) with an uncertainty of 2.2 arcmin in radius (90% C.L. statistical and systematic). 

The transient event lasts for nearly 170 seconds. The transient did not trigger the WXT onboard unit. The average 0.5-4 keV spectrum can be fitted by an absorbed power law with a photon index of 2.2 (+1.5/-1.1) with a column density of 0.2 (+0.4/-0.2) x 10^22 cm^-2. The derived average unabsorbed 0.5-4 keV flux is 1.2 (+1.2/-0.4) x 10^-9 erg/s/cm^2. The peak flux is around 4.4 x 10^-9 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. 

The Follow-up X-ray Telescope (FXT) on board the EP performed a ToO observation for EP251220a, starting at 2025-12-21T03:16:08 (UTC). Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 17.3160 deg, DEC = 19.0935 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic). 

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).