GCN Circular 41651
Subject
EP250901a: refined analysis of the EP-WXT and EP-FXT observations
Event
Date
2025-09-02T04:26:12Z (11 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Y. J. Song, T. Zhao, J. W. Hu, C. L. Guo, Z. X. Ling (NAO, CAS), on behalf of the Einstein Probe (EP) team:
The X-ray transient EP250901a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Song et al., GCN 41627), with several multi-wavelength follow-up observations (Lipunov et al., GCN 41629; Liu et al., GCN 41631; Evans et al., GCN 41644). The refined WXT data shows that the event started at T0=2025-09-01T12:02:50 (UTC) and lasted for about 150 s before the interruption of the autonomous follow-up. It has a peak flux of ~1 x 10^-9 erg/s/cm^2 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 1.1 x 10^21 cm^-2 and a photon index of 1.02 (-0.79/+0.84). The derived average unabsorbed 0.5-4 keV flux is around 5.05 (-1.54/+2.40) x 10^(-10) erg/s/cm^2.
The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 250s after T0. On-ground analysis of the FXT data found an fading source at R.A. = 273.0416, DEC = 18.7052 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), within the WXT error circle. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 3.0(-0.06/+0.07) x 10^21 cm^-2 and a photon index of 2.01(-0.15/+0.17). The derived average unabsorbed 0.5-10 keV flux is around 6.03 (-0.27/+0.43) x 10^(-12) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. A historical ROSAT detection (1RXS J181210.7+184228) lies 10.1 arcsec away from the FXT position, with a flux of 3 x 10^(-13) erg/s/cm^2 at 0.5-4 keV by assumimg a power-law spectrum with NH=3x10^20 cm^-2 and photon index =1.7.
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).