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EP J194531.8-612009: Einstein Probe detection of a new X-ray transient associated with a Cataclysmic Variable

ATel #17926; X. H. Tang (THU), K. R. Ni (CCNU), Y. J. Zhang (PKU), H. Sun (NAOC) on behalf of the Einstein Probe (EP) team
on 25 Jul 2026; 14:37 UT
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)

Subjects: X-ray, Cataclysmic Variable, Transient

We report the discovery of a new X-ray transient, EP J194531.8-612009, identified in the EP-WXT stacked data at RA = 296.419 deg, Dec =-61.318 deg, with an uncertainty of 2.2 arcmin in radius (90%, C.L statistical and systematic ). No known historical X-ray source is within the WXT error circle. The 0.5-4 keV flux estimated from the WXT stacked data is approximately 9.2x10^-13 erg/s/cm^2.

A follow-up observation with EP-FXT was carried out on 2026-07-25T03:40:13 (UTC) with an exposure of 1.9 ks, an X-ray source was detected within the WXT error circle at RA = 296.3825 deg, Dec =-61.3361 deg (uncertainty 10 arcsec, C.L statistical and systematic). The FXT light curve shows no significant variability. The X-ray spectrum can be fitted with an absorbed power law with absorption fixed at the Galactic value (N_H = 4.5 x 10^20 cm^-2) and a photon index of 1.40 (-0.15, +0.16). The unabsorbed 0.5-10 keV flux is 3.70 (-0.55, +0.60) x 10^-12 erg/s/cm^2. The eROSITA-DE gives a 0.2-2.3 keV flux upper limit of 8.5 x 10^-14 erg/s/cm2 at this position (from the eROSITA-DE upper limit server, TubÃn-Arenas et al. 2024; Merloni et al. 2024).

The position of FXT detection is consistent with that of the optical transient MASTER OT J194531.90-612009.5 (Balanutsa et al., ATel #8036), which is also identified as a Cataclysmic Variable (CV) in Gaia DR3 6442291862707764096 (Gaia Collaboration, 2022). Thus the X-ray transient is likely the outburst of the CV. The measured 0.5-10 keV flux corresponds to an X-ray luminosity of about 3.2 x 10^32 erg/s.

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).