TITLE:   GCN CIRCULAR
NUMBER:  45956
SUBJECT: EP261015a: Einstein Probe detection of a fast X-ray transient
DATE:    26/10/16 06:21:08 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

R.-Z. Li (YNAO), G. -L. Huang, Z.-X. Li (IHEP), M. Nie (YNAO) and H. Sun (NAOC) 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 EP261015a. The transient did not trigger EP-WXT onboard. We sent a GCN Notice manually with trigger ID: 11900947714. The WXT light curve of the transient began at 2026-10-15T13:15:36 UTC, exhibited two peaks, and lasted for 130 s. The WXT position of the source is R.A. = 44.341 deg, DEC = -13.365 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).

The 0.5-4 keV WXT  spectrum can be fitted by an absorbed powerlaw model with a fixed Galactic column density of 3.5 x 10^20 cm^-2 and a  photon index of 1.45 (-0.71,+0.88). The derived 0.5-4 keV unabsorbed flux is 6.25 (-2.91/+3.90) x 10^-10 erg/cm^2/s.

A follow-up observation with the Follow-up X-ray Telescope (FXT) has been scheduled. Further information will be updated when the telemetry data is received.

A possible optical counterpart to the transient was reported by NOT and LCO (Corcoran et al., GCN 45955).

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).