TITLE: GCN CIRCULAR NUMBER: 39833 SUBJECT: EP250321a: refined analysis of the EP-WXT and EP-FXT observations DATE: 25/03/22 09:11:25 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn
D. F. Hu (PMO, CAS), H. Q. Cheng (NAO, CAS), Z. H. Yang, Q. C. ZHAO (IHEP, CAS), L. Chen, W. D. Zhang (NAO, CAS) report on behalf of the Einstein Probe team:
The X-ray transient EP250321a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Hu et al., GCN 39800), and followed up by several telescopes (Lipunov et al., GCN 39803, Fu et al., GCN 39804, Perez-Garcia et al., GCN 39805, Brivio et al., GCN 39807, Zhu et al., GCN 39809, Becerra et al., GCN 39810, Page et al., GCN 39811, Pérez-Fournon et al., GCN 39812, Lee et al., GCN 39815, Ghosh et al., GCN 39816, Han et al., GCN 39817, Jin et al., GCN 39822, Zhu et al., GCN 39827, Gill et al., GCN 39831, Zheng et al., GCN 39832), with an optical counterpart detected at a redshift of 4.368 (Zhu et al., GCN 39809). Refined analysis of the WXT data shows that the event started at T0=2025-03-21T06:06:44.850 (UTC) and lasted for about 600s, with the tail of the fading light curve undetected due to the interruption of the observation. The peak flux approximately reached 4.2 x 10^(-9) erg/s/cm2 in the 0.5-4 keV band. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 4 x 10^20 cm^-2 and a photon index of 0.66 (-/+0.17). The derived average unabsorbed 0.5-4 keV flux is 1.73(-0.19/+0.21) x 10^(-9) erg/s/cm^2.
The Follow-up X-ray Telescope (FXT) on board EP observed this source about 16 ks after T0. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 179.2623, DEC = 17.3621 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), which is spatially consistent with the WXT transient. The FXT position was found to be consistent with the source detected by Swift-XRT (Page et al., GCN 39811) and optical/infrared counterparts as well. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 4 x 10^20 cm^-2 and a photon index of 1.98 (-/+0.06). The derived average unabsorbed 0.5-10 keV flux is 9.23 (-0.54/+0.58) x 10^(-12) erg/s/cm^2 with an exposure time of about 2990 seconds.
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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