GCN Circular 43817

Q. C. Liu (THU), J. W. Hu and H. Sun (NAO, CAS) on behalf of the Einstein Probe (EP) team:

We report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP260223a. The transient did not trigger the WXT onboard unit due to the limited number of counts. The WXT position of the source is R.A. = 92.774 deg, DEC = 56.577 deg (J2000) with an uncertainty of 2.7 arcmin in radius (90% C.L. statistical and systematic). The preliminary analysis of the WXT data shows that the event started at T0=2026-02-23T11:20:27 (UTC), and lasted for approximately 130 seconds. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 1.7×10^21 cm^-2 and a photon index of 2.6 (+1.1/-0.9). The derived average unabsorbed 0.5-4 keV flux is 4.1 (+1.7/-1.3) x 10^-10 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

Further FXT follow-up observations have been arranged.

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).