GCN Circular 45035
Subject
EP260623a:refined analysis of the EP-WXT and EP-FXT observation
Event
Date
2026-06-24T09:06:30Z (3 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
B.T. Wang (YNAO, CAS), Y. L. Hua (PMO, CAS) and C. C. Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP260623a triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Wang et al., GCN 45020) and followed by several optical telescopes (Li et al., GCN 45021; Angulo et al., GCN 45024; Brivio et al., GCN 45025; Jiang et al., GCN 45026; Li et al., GCN 45027; Jelinek et al., GCN 45029; Lee et al., GCN 45030; Saikia et al., GCN 45034) with the redshift confirmed at z=0.703 (Izzo et al., GCN 45022). The refined analysis of the WXT data shows that the event started at T0=2026-06-23T02:52:27Z (UTC) and lasted for 103 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*ztbabs*powerlaw) with a fixed Galactic hydrogen column density of 8.1 × 10^20 cm^-2 and a fixed redshift of z = 0.703. The best-fit photon index is 1.83 (-0.56/+0.63), and the intrinsic hydrogen column density is 2.25 (-0.97/+1.21) × 10^22 cm^-2. The derived average unabsorbed 0.5–4 keV flux is 3.17 (-0.74/+1.39) × 10^-9 erg/s/cm^2. The estimated peak flux is 1.0 × 10^-8 erg/s/cm^2, corresponding to a peak luminosity of 2.2 × 10^49 erg/s.
The autonomous observation by the Follow-up X-ray Telescope (FXT) was performed at 2026-06-23T02:56:08 (UTC), about 3.7 minutes after T0. The exposure time of this observation is 3211 s. The on-ground analysis shows that an uncatalogued source was detected at R.A., Dec. = 328.2698, 12.7938 deg (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). The FXT light curve displays two pulses structure lasting for around 210 s. 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 8.1 × 10^20 cm^-2 and a photon index of 1.96 (-/+ 0.03), and the intrinsic hydrogen column density is 0.94 (-/+ 0.04) × 10^22 cm^-2. The derived average unabsorbed 0.5-10 keV flux is 7.33 (-/+ 0.10) × 10^-9 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).