GCN Circular 43296
Subject
EP260101a: refined analysis of the EP-WXT and EP-FXT observations
Event
Date
2026-01-02T04:56:23Z (18 hours ago)
Edited On
2026-01-02T15:22:21Z (8 hours ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
M.-H. Zhang, Y.-L. Wang, W. Chen, H. Sun (NAO, CAS) on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP260101a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Zhang et al., GCN 43288). The refined analysis of the WXT data shows that the event started at T0=2026-01-01T06:31:46 (UTC) and lasted for 200 seconds before the observation was interrupted by the autonomous follow-up. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 1.3 x 10^21 cm^-2 and a photon index of 1.7 (-0.7/+0.7). The derived average unabsorbed 0.5-4 keV flux is 6.6 (-2.0/+3.2) x 10^(-11) erg/s/cm^2.
The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously at 2026-01-01T06:38:27 (UTC), about 7 minutes after T0. The exposure time of this observation is 4674s. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 46.0631, DEC = 39.4376 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent with the WXT position. 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 1.3 x 10^21 cm^-2 and a photon index of 2.9 (-0.3/+0.4). The derived average unabsorbed 0.5-10 keV flux is 8.4 (-1.2/+1.3) x 10^(-13) erg/s/cm^2.
We performed another ToO observation with EP-FXT, starting at 2026-01-01T11:08:28 (UTC), with an exposure time of 6950 s. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with the same fixed Galactic hydrogen column density and a photon index of 3.5 (-1.0/+1.1). The derived average unabsorbed 0.5-10 keV flux is 8.6 (-2.9/+4.1) x 10^(-14) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
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).