TITLE: GCN CIRCULAR NUMBER: 39448 SUBJECT: EP250223a: refined analysis of the EP-WXT and FXT observations DATE: 25/02/24 10:08:07 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn
Y. Wang (PMO, CAS), T. Y. Lian, W. J. Zhang (NAOC, CAS), X. Tian (GXU), R. Z. Li(YNAO, CAS), H. W. Pan (NAOC, CAS) report on behalf of the Einstein Probe team:
The X-ray transient EP250223a triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Lian et al., GCN 39429), and followed by the Swift XRT (Kennea et al., GCN 39437) and several optical telescopes (Levan et al., GCN 39436, Wu et al., GCN 39439, Pérez-Fournon et al., GCN 39440, Izzo et al., GCN 39441, Xin et al., GCN 39445) at the redshift of 2.756 (Levan et al., GCN 39438). The refined analysis of the WXT data shows that the event started at 2025-02-23T15:02:05.66 (UTC) and lasted for 140 s with the peak flux of 2 x 10^(-9) erg/s/cm^2, 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 equivalent hydrogen column density of 1.36 x 10^21 cm^-2 and a photon index of 2.1 (-/+0.6). The derived average unabsorbed 0.5-4 keV flux is 4.4 (-1.1/+1.4) x 10^(-10) erg/s/cm^2.
The autonomous observation by the Follow-up X-ray Telescope (FXT) was performed about two minutes later. The on-ground analysis shows that an uncatalogued source was detected at R.A. = 98.2748, DEC = -22.4443 (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 Galactic equivalent hydrogen column density of 1.36 x 10^21 cm^-2 and a photon index of 1.97 (-/+0.05). The derived average unabsorbed 0.5-10 keV flux is 2.5 (-/+0.1) x 10^(-11) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
Further follow-up observations with EP-FXT have been scheduled.
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).
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