EP260324a: Einstein Probe detection of an X-ray transient
ATel #17728; Q. Y. Wu (NAO, CAS), C. Y. Dai (NJU), B. T. Wang (YNAO, CAS), C. Y. Wang (THU), H. Z. Wu (HUST), H. W. Pan (NAO, CAS) on behalf of the Einstein Probe (EP) team
on 26 Mar 2026; 14:17 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 during an observation on 2026-03-24T07:04:20 (UTC), designated EP260324a. Analysis of this earliest WXT observation shows that its spectrum can be fitted with a power-law model, with the absorption column density fixed at the Galactic value of 3 x 10^20 cm^-2. The best-fit photon index is 1.4 (-0.4/+0.4), and the unabsorbed flux in the 0.5-4 keV band is 3.9 (-1.0/+1.2) x 10^-11 erg/s/cm^2. The transient was subsequently detected in multiple WXT observations over the following ~1 day. In these detections, its X-ray flux were similar to the earliest one, while the spectrum exhibited a gradual softening with time.
A follow-up observation with the Follow-up X-ray Telescope (FXT) on-board EP was performed on 2026-03-25T07:06:16 (UTC), with an exposure time of 2.6 ks. Analysis of the telemetry data reveals an uncatalogued X-ray source within the WXT error circle, located at R.A., Dec. = 131.0547, 82.0202 (J2000), with an uncertainty radius of 10 arcsec (90% C.L.). Its spectrum can be fitted with a power-law model, with the absorption column density fixed at the Galactic value of 3 x 10^20 cm^-2. The best-fit photon index is 2.64 (-0.02/+0.02), and the unabsorbed flux in the 0.5-10 keV band is 3.94 (-0.04/+0.04) x 10^-11 erg/s/cm^2.
The FXT source, with a positional uncertainty radius of 10 arcsec, is located 2.4 arcsec from a galaxy SDSS J084412.34+820110.9 at R.A., Dec. = 131.05146, +82.01970 (J2000), for which photometric redshifts of 0.095 and 0.082 have been reported in GLADE+ and DESI DR9, respectively. Assuming this association, the observed FXT flux corresponds to a 0.5-10 keV X-ray luminosity of approximately 9 x 10^44 erg/s or 7 x 10^44 erg/s, respectively.
Further FXT follow-up observations have been arranged. And multi-band 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).
