GCN Circular 45332
Subject
EP260809a:refined analysis of the EP-WXT and autonomous EP-FXT observation
Event
Date
2026-08-09T17:58:56Z (a day ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
X. X. Sun(NAO, CAS), Q. J. Huang (PMO, CAS), K. J. Zhang(YNU), J. P. Chen (SYSU), Z. X. Ling(NAO, CAS) on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP260809a triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Sun et al., GCN 45321) and followed by several optical telescopes (Adami et al., GCN 45324; Jiang et al., GCN 45325; Basa et al., GCN 45326 ) . The refined analysis of the WXT data shows that the event started at T0=2026-08-09T00:38:21(UTC) and lasted for 34 s before the observation was interrupted by the autonomous follow-up observation.The average WXT 0.5–4 keV spectrum can be fitted with an absorbed power-law model (tbabs*tbabs*cflux*powerlaw), with the Galactic hydrogen column density fixed at 8.24 × 10^20 cm^-2. The best-fit photon index is 1.74 (-0.63/+0.88), and the additional hydrogen column density is 1.25 (-1.25/+2.23) × 10^21 cm^-2. The derived average unabsorbed 0.5–4 keV flux is 3.32 (-0.77/+0.98) × 10^-9 erg s^-1 cm^-2.
The autonomous observation by the Follow-up X-ray Telescope (FXT) was performed at 2026-08-09T01:25:57(UTC), about 47.6 minutes after T0. The exposure time of this observation is 3020 s. The on-ground analysis shows that an uncatalogued source was detected at R.A., Dec. = 328.5791, 17.6054 deg (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic equivalent hydrogen column density of 8.24 × 10^20 cm^-2 and a photon index of 2.00 (-0.28/+0.29), and the intrinsic hydrogen column density is 0.08 (-/+0.08) × 10^22 cm^-2. The derived average unabsorbed 0.5-10 keV flux is 1.27(-0.12/+0.14)× 10^(-12) 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).