GCN Circular 41841

Q. Y. Wu (NAO, CAS), C. Y. Dai (NJU), D. Y. Li, H. W. Pan (NAO, CAS) on behalf of the Einstein Probe (EP) 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 through ground data analysis, designated EP250916a. The WXT position of the source is R.A. = 268.279 deg, DEC = -35.317 deg (J2000) with an uncertainty of 2.4 arcmin in radius (90% C.L. statistical and systematic), located at low Galactic latitude (l = 355.331, b = -4.634).

The source was detected in three consecutive observations: the first started at 2025-09-16T03:33:13 (UTC) with a flux of ~3e-11 erg/s/cm^2; two subsequent observations, approximately 2 h and 6 h later, also detected the source with slightly higher fluxes of ~5e-11 erg/s/cm^2. As of the latest observation, the transient has persisted for ~7 hours. A preliminary spectral analysis shows that the 0.5-4 keV spectrum can be fitted by an absorbed power-law model with Galactic hydrogen column density fixed at 3.76 x 10^21 cm^-2 and a photon index of 1.1 (-/+0.8). No known X-ray sources have been found previously within the EP-WXT error circle. A stellar flare origin cannot be excluded at this stage. 

Follow-up observations with the Follow-up X-ray Telescope (FXT) on board EP have been scheduled, and further information will be updated when the telemetry data are received.

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).