GCN Circular 43349

Subject

EP260105b/EP-WXT trigger 01709251028: Einstein Probe detection of an X-ray transient

Event

EP260105b

Date

2026-01-07T13:08:28Z (4 days ago)

From

EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

Via

Web form

X. X. Sun, G. J. Yang, M. J. Liu, , C. C. Jin, Y. Liu (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, designated EP260105b. The transient triggered EP-WXT (ID: 01709251028) at 2026-01-05T18:47:36 (UTC). The WXT position of the source is R.A. = 158.387 deg, DEC = 17.616 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).

In a previous Circular (Yang et al. , GCN 43340Loading...
 
 ) , this event was tentatively classified as a stellar flare associated with ET Leo, based on the preliminary localization and flux estimation available at that time. After refined data processing and ground analysis of the WXT and FXT observations, we find that the evnet is unlikely to be a stellar flare from ET Leo. We find a short-timescale X-ray varability in the WXT refined telemetry data,  therefore classify this event as an X-ray transient. 

A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 158.3695 deg, DEC = 17.6123 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). A significant decay was detected in the light curve during the initial phase of FXT observation. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 2.97 x 10^20 cm^-2 and a photon index of 2.18(-0.87,0.92). The derived average unabsorbed 0.5-10 keV flux is 2.6 (-0.94/+2.59) x 10^(-13) erg/s/cm^2. 

Further multiwavelength follow-up observations are encouraged to clarify the physical nature of this transient.

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).