GCN Circular 45959

G. -L. Huang, Z.-X. Li (IHEP), R.-Z. Li, M. Nie (YNAO) and H. Sun (NAOC) on behalf of the Einstein Probe (EP) team:

We report the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP261015c. The EP-WXT observation (ID: 11900946826) started at 2026-10-15 17:29:32 (UTC), with an exposure time of 1200 s. The WXT light curve of the transient began at 2026-10-15T17:37:52 UTC lasted for 700 s. The WXT position of the source is R.A. = 21.920 deg, DEC = 66.601 deg (J2000) (l=126.526 deg, b=3.983 deg) with an uncertainty of 2.695 arcmin in radius (90% C.L. statistical and systematic).

We fitted the WXT spectrum with an absorbed power-law model, fixing nH at 0.581e22 cm-2. We obtained a spectral index of 4.3 (-1.8/+1.9) and an unabsorbed flux of 1.5 (-0.6/+1.2) e-10 erg/cm2/s during the flare. Over the entire observation, the spectral index was 4.4 (-1.8/+1.9), with an unabsorbed flux of 9.0 (-3.7/+6.9) e-11 erg/cm2/s.

A follow-up observation by EP-FXT have been scheduled. Multi-wavelength observations are strongly encouraged.

Launched on January 9, 2024, EP is a space X-ray observatory designed to monitor the soft X-ray sky with rapid X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE, and CNES.