TITLE:   GCN CIRCULAR
NUMBER:  45332
SUBJECT: EP260809a:refined analysis of the EP-WXT and autonomous EP-FXT observation
DATE:    26/08/09 17:58:56 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

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).