GCN Circular 43424

C. Wang, Z. X. Li, G. L. Huang, J. Y. Cao (IHEP), Y. H. Jiang (NJU), Z. X. Ling (NAOC) 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 EP260116a. A significant outburst is observed in the WXT light curve. The transient triggered EP-WXT (ID: 01709251336) at 2026-01-16T00:08:51 (UTC). The WXT position of the source is R.A. = 113.768 deg, DEC = 6.329 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).

The average WXT 0.5 - 4 keV spectrum can be fitted with an absorbed power law with a hydrogen column density of 7.65 x 10^20 cm^-2 and a photon index of 1.77. The derived average unabsorbed 0.5-4 keV flux is 7.16 x 10^-11 erg/s/cm^2. 

A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically with a 24-minute delay, starting at 2026-01-16 00:32:38 (UTC), by which time the outburst had already ended. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 113.7617 deg, DEC = 6.3244 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic).

The average FXT 0.5 - 10.0 keV spectra can also be fitted with an absorbed power law with the same hydrogen column density and a photon index of 1.98. The derived average unabsorbed 0.5-10.0 keV flux is 9.52 x 10^-14 erg/s/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).