EP J075040.5+662346: Einstein Probe detection of an X-ray transient associated with the CV ZTF18abujfcu
ATel #17578; D. Y. Li (NAO, CAS), Z. C. Zou (NJU), H. Zhou (PMO, CAS), Y. Liu, Z. X. Ling (NAO, CAS) on behalf of the Einstein Probe (EP) team
on 6 Jan 2026; 09:34 UT
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)
Subjects: X-ray, Cataclysmic Variable
We report the detection of an X-ray transient, designated EP J075040.5+662346, with the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The source was identified through stacking of WXT data, and remained at a relatively constant flux level from October 2025 to January 2026. The WXT position of the source is R.A. = 117.650 deg, Dec. = 66.393 (J2000) with an uncertainty of 2.1 arcmin in radius (90% C.L. statistical and systematic). The estimated 0.5-4 keV flux derived from the WXT spectrum is ~5 x 10^(-12) erg/s/cm^2. Due to solar constraints, it was not visible to WXT between April and September 2025. Stacking of the pre-April WXT data shows no significant emission.
A target-of-opportunity follow-up observation was performed with the Follow-up X-ray Telescope (FXT) starting at 2026-01-05 09:43:12 (UTC). Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 117.6688 deg, DEC = 66.3960 deg (J2000), with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). The FXT position lies around 2.5 arcsec away from the known cataclysmic variable ZTF18abujfcu. The FXT light curve exhibits significant short-term variability. The average 0.5-10 keV spectrum can be fitted with a power law model with a photon index of 1.34 (-0.08/+0.08). No significant absorption was required. The derived average 0.5-10 keV flux is 1.10 (-0.09/+0.10) x 10^(-11) erg/s/cm^2. The quoted errors are at 90% confidence level.
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).
