GCN Circular 43693

Y.J. Zhang (THU), R. Shi (PMO), Z. H. Yang, Q. C. Zhao (IHEP), G. Y. Zhao (SYSU) and Z. X. Ling (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 EP260209a. The EP-WXT observation started at 2026-02-09T13:44:06 (UTC). The WXT position of the source is R.A. = 159.683 deg, DEC = 28.312 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The light curve exhibits a flare in the first 50 seconds of the WXT observation, with a 0.5--4 keV flux of approximately 7.4 × 10^-10 erg s^-1 cm^-2. The delayed report of the transient is due to its relatively low detection significance.

We performed a follow-up observation with the Follow-up X-ray Telescope (FXT), starting at 2026-02-10T07:02:41 (UTC), about 17 hours after the detection. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 159.6834 deg, DEC = 28.3078 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). The uncatalogued X-ray source has a 0.5--10 keV flux of 1.1 x 10^-13 erg s^-1 cm^-2, with a powerlaw index of 1.3 and a fixed galactic NH value (1.6 x 10^20 cm^-2).

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).