GCN Circular 45305
Subject
EP260806a: refined analysis of the EP-WXT and autonomous EP-FXT observations
Event
Date
2026-08-07T09:57:42Z (2 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
R.-Z. Li (YNAO, CAS), Y.-H. Cheng (YNU), G.-J. Yang and W.-D. Zhang (NAOC) on behalf of the Einstein Probe (EP) team:
The X-ray transient EP260806a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Li et al., GCN 45299), and followed up by LCO (He et al., GCN 45300), GTC (de Ugarte Postigo et al., GCN 45302), Liverpool Telescope (Eyles-Ferris et al., GCN 45303) and SVOM/VT (Ma et al., GCN 45304). We report the refined analysis of the EP-WXT observation during which the source was detected, and the autonomous EP-FXT follow-up observation.
The WXT observation in which the source was detected started at 2026-08-06 22:57:37 (UTC) and lasted for ~290 seconds. No obvious variability or flaring activity of the source was seen in its lightcurve during the WXT observation. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 4.64 x 10^20 cm^-2, an intrinsic column density of 1.17 (+/- 0.51) x 10^18 cm^-2 and a photon index of 1.94 (-0.81/+2.7). The derived average unabsorbed 0.5-4 keV flux is 1.65 (-0.58/+5.7) x 10^-10 erg s^-1 cm^-2.
The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously starting at 2026-08-06T23:03:51 (UTC, 374 seconds after the first WXT observation). The exposure time of this observation was 3950 s. On-ground analysis of the FXT data found an uncatalogued X-ray source at R.A. = 12.3066 deg, DEC = 35.5603 deg (J2000), with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent with the WXT position.
The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 4.64 x 10^20 cm^-2, an intrinsic column density of 1.92 (+/-0.13) x 10^21 cm^-2 and a photon index of 2.02 (+/-0.04). The derived average unabsorbed 0.5-10 keV flux is 2.13 (+/-0.03) x 10^-10 erg s^-1 cm^-2. The peak luminosity is estimated to be 3.8 × 10^45 erg s^-1 in the 0.5-10 keV energy band, adopting a redshift of z = 0.089 (de Ugarte Postigo et al., GCN 45302). The source flux had been declining during the FXT observation.
The uncertainties are at the 90% confidence level for the above spectral parameters.
Further follow-up observations with EP/FXT are scheduled.
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).