TITLE: GCN CIRCULAR NUMBER: 39556 SUBJECT: EP250302a: Einstein Probe detection of an X-ray transient DATE: 25/03/03 07:23:40 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> C. Y. Dai (NJU), Y.Q. Zhao (USTC, PRIC), X. Mao, S. X. Wen, W. Yuan (NAO, CAS) report on behalf of the Einstein Probe team:   We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250302a. The transient triggered EP-WXT (ID: 01709132186) at 2025-03-02T15:36:04 (UTC). The light curve shows a burst lasting at least 30s, with a peak of about 9 x 10^(-9) erg/s/cm^2. The spectrum can be fitted by an absorbed power-law with a photon index of 0.6(-0.4/+0.4) (with the column density fixed at the Galactic one of 2.36 x 10^20 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 7.0(-1.6, +2.0) x 10^(-9) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error cirlce, an uncatalogued X-ray source was detected at R.A. = 169.5151 deg, DEC = 33.5851 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). A secondary burst is detected starting at 2025-03-02T15:56:06 (UTC) and last for ~500 s, the spectrum during the flare can be fitted by an absorbed power-law with a photon index of 1.8(-0.05/+0.0.5) (with the column density fixed at the Galactic one of 2.36 x 10^20 cm^-2), and the derived average unabsorbed 0.5-10 keV flux is 9.6(-0.4, +0.4) x 10^(-11) erg/s/cm^2. Additionally, we note that the optical counterparts (GCN 39550, GCN 39551, GCN 39553, and GCN 39555) lie within the positional uncertainty of EP-FXT. Further follow-up observations are encouraged to identify the nature of this X-ray transient. 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). View this GCN Circular online at https://gcn.nasa.gov/circulars/39556. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...