TITLE:   GCN CIRCULAR
NUMBER:  44324
SUBJECT: EP260416a: refined analysis of the EP-WXT and autonomous EP-FXT observations
DATE:    26/04/17 09:10:30 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

D. F. Hu (PMO, CAS), J. Yang (ZZU), M. J. Liu, Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team: 

The X-ray transient EP260416a was detected by the Wide-field X-ray Telescople (WXT) on board the Einstein Probe (EP) mission (Hu et al., GCN 44307), and followed by several optical telescopes (Ducoin et al., GCN 44310, Li et al., GCN 44315, Hua et al., GCN 44318, Lee et al., GCN 44319, Zheng et al., GCN 44321). The refined analysis of the WXT data shows that the event was detected at the beginning of the observation starting at T0=2026-04-16T05:25:24 (UTC), and lasted for about 250 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 with a fixed Galactic hydrogen column density of 1.02 × 10^20 cm^-2, and a photon index of 1.0 +/- 0.8. The derived average unabsorbed 0.5-4 keV flux is 3.2 (-1.4, +2.7) × 10^(-10) erg/s/cm^2.

The autonomous observation by the Follow-up X-ray Telescope (FXT) was performed at 2026-04-16T05:33:14 (UTC), about 8 minutes after T0. The exposure time of the observation is 2.4 ks. The on-ground analysis of the FXT data found an uncatalogued source at R.A. = 198.8351, DEC = 31.5972 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The FXT light curve shows no significant variability. The average FXT 0.5-10 keV spectra can be fitted with an absorbed power-law with a free hydrogen column density of 1.62 +/- 0.18 × 10^21 cm^-2, and a photon index of 2.14 +/- 0.08. The derived average unabsorbed 0.5-10 keV flux is 3.4 (-0.1, +0.1) × 10^(-10) erg/s/cm^2. No significant variability is observed from T0 to T0+6 ks. The uncertainties are at the 90% confidence level for the above parameters. 

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).