GCN Circular 39738
Subject
EP250315a: refined analysis of the EP-WXT and EP-FXT observations
Date
2025-03-15T15:05:35Z (a day ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
H.Sun (NAO, CAS), J. Q. Peng (IHEP, CAS), M. Q. Huang (USTC), W. X. Wang, W. D. Zhang (NAO, CAS) report on behalf of the Einstein Probe team:
The X-ray transient EP250315a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Peng et al., GCN 39731). An optical counterpart candidate was detected by Gemini North (Quirola-Vasquez et al, GCN 39733). Refined analysis of the WXT data shows that the event started at T0=2025-03-15T05:58:16 (UTC) and lasted for about 280 s before the observation was disrupted by the autonomous follow-up observation. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 2.97 x 10^20 cm^-2 and a photon index of 0.36 (-/+0.50). The derived average unabsorbed 0.5-4 keV flux is 5.4(-1.7, +2.2) x 10^(-10) erg/s/cm^2.
The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 6 min after T0, starting at 2025-03-15T06:04:15 (UTC). Within the WXT error circle, on-ground analysis of the FXT data found an uncatalogued source at R.A. = 203.0287, DEC = -9.5301 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic), which is also consistent with the position of the optical counterpart candidate. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 2.97 x 10^20 cm^-2 and a photon index of 1.80(-/+0.05). The derived average unabsorbed 0.5-10 keV flux is 3.7(-/+0.17) x 10^(-11) 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).