GCN Circular 46235
Subject
EP261111a: Refined analysis of EP-FXT observations
Event
Date
2026-11-12T09:58:53Z (2 hours ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Z.X. Li, G. L. Huang (IHEP, CAS), Y. J. Zhang (THU), W. D. Zhang (NAO, CAS) on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP261111a triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission at 2026-11-11T13:25:10 (UTC) (Huang et al., GCN 46226). Subsequent optical follow-up observations were reported by Jin et al. (GCN 46227) and Buckley et al. (GCN 46230).
The refined analysis of the WXT data shows that the event started at T0 = 2026-11-11T13:24:09 (UTC), and lasted for approximately 60 seconds, after which the WXT light curve was interrupted due to the autonomous follow-up observation. During the flare, the average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density (NH) of 1.10×10^21 cm^-2, and a photon index of 1.23 (-0.64/+0.67). The derived average unabsorbed 0.5-4 keV flux is 1.30 (-0.43/+0.65) ×10^(-9) erg/s/cm^2.
The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously at 2026-11-11T13:28:25 (UTC, T0+256s). The exposure time of this observation is 3150 s. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 43.1193, DEC = 44.4351 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic) within the WXT error circle. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 1.10×10^21 cm^-2 and a photon index of 1.90 (-0.14/+0.15). The derived average unabsorbed 0.5-10 keV flux is 3.15 (-0.36/+0.41)×10^(-12) erg/s/cm^2. All uncertainties are reported at the 90% confidence interval.
Further FXT follow-up observations have been arranged.
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).