Einstein Probe detection of an X-ray transient EP250315b
ATel #17083; D. Y. Li (NAOC, CAS), Q. C. Shui (IHEP, CAS), M. Q. Huang (UCST), Y. Liu, W. Yuan (NAOC, CAS) on behalf of the Einstein Probe team
on 18 Mar 2025; 04:19 UT
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)
We report on the detection of an X-ray transient designated EP250315b (EP J163933.2-411414), by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient was first detected by WXT at 2025-03-15T10:47:55 (UTC). We conducted a follow-up observation with the Follow-up X-ray telescope (FXT) on board EP which started at 2025-03-17T00:35:30 (UTC), about 38 hours after the WXT detection. The observation, with an exposure time of 1.4 ks, shows an uncatalogued X-ray source located at R.A. = 249.8884 deg, DEC = -41.2372 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). The Galactic coordinate of source is l = 342.1414 deg, b = 3.6492 deg. The average 0.5-10 keV spectrum can be fitted by an absorbed power law with a photon index of 1.6 (+/-0.1), and an absorption column of 0.43 (+/- 0.05) x 10^22 cm^-2, giving an unabsorbed 0.5-10 keV flux of 1.10 (+/0.05) x 10^-10 erg/s/cm2. The uncertainties are at the 90% confidence level for the above parameters. The FXT spectral fitting results are consistent with that in the WXT detection.
No known X-ray sources were found within the error circle of FXT position. Continuous monitoring of this new transient will be performed with EP-FXT in the following weeks. Swift and NICER observations have also been requested. More multi-wavelength follow-up observations are encouraged to explore the nature of this new source.
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). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.
