TITLE:   GCN CIRCULAR
NUMBER:  41693
SUBJECT: EP250903a: refined analysis of the EP-WXT and EP-FXT observations
DATE:    25/09/04 11:34:49 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

Y. J. Zhang (THU), T. Zhao, Y. J. Song, H. Sun(NAO, CAS), on behalf of the Einstein Probe (EP) team:

The X-ray transient EP250903a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Zhao et al., GCN 41674), with several multi-wavelength follow-up observations (O'Neill et al., GCN 41672; An et al., GCN 41675; Lipunov et al., GCN 41676; Levan et al., GCN 41680; Xin et al., GCN 41683). The refined WXT data shows that the event started at T0=2025-09-03T13:59:40 (UTC) and lasted for about 200 s before the interruption of the autonomous follow-up. WXT detected the source during its rising phase with a peak flux of 6 x 10^(-10) erg/s/cm^2. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 1.0 x 10^20 cm^-2 and a photon index of 1.6 (+/-0.6). The derived average unabsorbed 0.5-4 keV flux is around 1.0 (-0.5/+1.1) x 10^(-10) erg/s/cm^2. 

The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 428s after T0. On-ground analysis of the FXT data found an fading source at R.A. = 339.1101, DEC = -49.8541 (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 1.0 x 10^20 cm^-2 and a photon index of 2.6(+/-0.1). The derived average unabsorbed 0.5-10 keV flux is around 4.3(+/-0.3) x 10^(-12) 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).