TITLE:   GCN CIRCULAR
NUMBER:  44222
SUBJECT: EP260405a: Einstein Probe detection of a fast X-ray transient
DATE:    26/04/06 04:04:37 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

R.-Z. Li (YNAO), Y. J. Yi (BNU), H.Q. Cheng and Z.-X. Ling (NAOC) on behalf of the Einstein Probe (EP) team:

We report on the detection of a fast X-ray transient, designated EP260405a by the Wide-field X-ray Telescope (WXT) on the Einstein Probe (EP) mission (EP/WXT trigger ID: 11900672904), and followed by COLIBRI (Lincetto et al. GCN #44220). The X-ray flare started at T0 = 2026-04-05T03:46:27 (UTC) with a duration of about 150s. The WXT position of the source is R.A. = 184.423 deg, DEC = -20.156 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The WXT spectrum can be fitted by an absorbed powerlaw model with a fixed Galactic column density of 4.83 x 10^20 cm^-2, and a photon index of 2.0 (-0.4, +0.4). The 0.5-4 keV unabsorbed flux is 4.2 (-0.9, +1.2) x 10^-10 erg/cm^2/s.
 
A follow-up observation with the Follow-up X-ray Telescope (FXT) was conducted, beginning at 2026-04-05T16:23:25 (UTC) with an exposure time of 3ks. Within the EP/WXT error circle, no significant X-ray source was detected in the EP/FXT data, yielding an upper limit of 5 x 10^-14 erg/cm^2/s (90% C.L.). 

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).