GCN Circular 44814

Y. F. Liang (PMO), J. W. Hu, Y. L. Wang, Z. X. Ling(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 EP260603a. The ground analysis of the WXT data shows that the event started at 2026-06-03T17:51:12 (UTC) and lasted for about 350 s. The WXT position of the source is R.A. = 229.721 deg, DEC = -26.940 deg (J2000) with an uncertainty of 2.8 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 fixed Galactic hydrogen column density of 1.56 × 10^21 cm^-2, and a photon index of 1.11 (-0.77, +0.80). The derived average unabsorbed 0.5-4 keV flux is 1.2 (-0.4, +0.6)×10^(-10) erg/s/cm^2. The peak flux is about 9.5×10^(-10) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

A Target-of-Opportunity observation with the Follow-up X-ray Telescope (FXT) has been scheduled. Further information will be updated when the telemetry data is received.
Further multi-wavelength observations are encouraged to determine the nature of this source.

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).