EP250916a: Einstein Probe detection of an X-ray transient
ATel #17395; C. Y. Dai (NJU), Q. Y. Wu, D. Y. Li, H. W. Pan (NAO, CAS) on behalf of the Einstein Probe (EP) team
on 18 Sep 2025; 07:46 UT
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)
Subjects: X-ray, Transient
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 through ground data analysis, designated EP250916a. The WXT position of the source is R.A. = 268.279 deg, DEC = -35.317 deg (J2000) with an uncertainty of 2.4 arcmin in radius (90% C.L. statistical and systematic), located at low Galactic latitude (l = 355.331, b = -4.634).
This source keeps brightening in the WXT monitoring, with the flux increasing from approximately 3 x 10^-11 erg/s/cm^2 at 2025-09-16 03:33:13 to about 2 x 10^-10 erg/s/cm^2 at 2025-09-17 17:54:10 (UTC). A preliminary spectral analysis indicates that the 0.5-4 keV spectrum can be fitted with an absorbed power-law model, with the Galactic hydrogen column density fixed at 3.76 x 10^21 cm^-2 and a photon index of 1.1 (+/- 0.8). No previously known X-ray sources have been reported within the EP-WXT error circle.
EP-FXT performed a follow-up observation of EP250916a at 2025-09-17 11:34:24 (UTC), about 32 hours after the WXT detection, with an exposure time of 3.1 ks. FXT detected a bright source within the WXT error circle at R.A. = 268.2379 and Dec. = -35.3222 (J2000), with a positional uncertainty of 10 arcseconds (radius, 90% confidence level, including both statistical and systematic errors). The fit with a power-law spectrum yields Nh = 3.1 x 10^21 cm^-2 and a photon index of 1.66 +- 0.02, with a 0.5-10 keV flux of 5.4 x 10^-10 erg/s/cm^2.
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).
