GCN Circular 46014

H. N. Yang (NAO,CAS), Z. X. Li (IHEP), M. Y. Wang (XMU), Q. Y. Wu, H. W. Pan (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 EP261023a. The transient triggered EP-WXT (ID 01709321471) at 2026-10-23 04:03:19 (UTC). The WXT position of the source is R.A. = 3.994 deg, DEC = 51.589 deg (J2000) with an uncertainty of 2.749 arcmin in radius (90% C.L. statistical and systematic). The transient starts from T0 = 2026-10-23 04:02:14 (UTC), with the source remaining bright for ~64 s until the end of this observation. Analysis of this earliest WXT observation shows that its spectrum can be fitted with a power-law model, with the absorption column density fixed at the Galactic value of 2e21 cm^-2. The best-fit photon index is 1.3(+/-0.5), and the unabsorbed flux in the 0.5-4 keV band is 2.0(-0.4/+0.6) e-9 erg/s/cm^2. 

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. = 3.9824 deg, DEC = 51.5984 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). we jointly fitted the FXT-A and FXT-B spectra in the 0.5–10 keV band with an absorbed power-law model. The hydrogen column density was fixed at the Galactic value. The best-fit photon index is 2.6(+/-0.5). The average unabsorbed 0.5-10 keV flux is 8.5(+2.2/-1.8) e-13 erg/s/cm^2. Within the position error of this source, there is only one star, Gaia DR3 395023397938295040, located at a distance of 1.5 kpc. The association between the FXT source and the WXT trigger remains uncertain.

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).