TITLE:   GCN CIRCULAR
NUMBER:  46014
SUBJECT: EP261023a: Einstein Probe detection of an X-ray transient
DATE:    26/10/23 14:56:22 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

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).