TITLE: GCN CIRCULAR NUMBER: 39738 SUBJECT: EP250315a: refined analysis of the EP-WXT and EP-FXT observations DATE: 25/03/15 15:05:35 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

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).

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