EP J185209.1-192017: Einstein Probe detection of a new X-ray transient

ATel #17922; D. Y. Li (NAO, CAS), Z. X. Li (IHEP, CAS), A. Li (BNU), C. Y. Wang (THU), H. W. Pan (NAO, CAS) on behalf of the Einstein Probe (EP) team
on 24 Jul 2026; 05:47 UT
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)

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We report the discovery of a new X-ray transient, EP J185209.1-192017, identified in the EP-WXT stacked data at RA = 283.026 deg, Dec =-19.327 deg, with an uncertainty of 2.2 arcmin in radius (90%, C.L statistical and systematic ). The source is at low Galactic latitude (l = 15.666, b = -8.821) and was firstly detected in the stacked data around May 2026. 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 a few times 10^-12 erg/s/cm^2.

A follow-up observation with EP-FXT was carried out on 2026-07-20T20:12:51 (UTC) with an exposure of 1.9 ks, an X-ray source was detected within the WXT error circle at RA = 283.0379 deg, Dec =-19.3381 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 = 2.0 x 10^21 cm^-2) and a photon index of 2.5 (-0.2, +0.2). The unabsorbed 0.5-10 keV flux is 2.0 (-0.3, +0.3) x 10^-12 erg/s/cm^2. The ROSAT all-sky survey gives a 0.2-2 keV flux upper limit of 2.8 x 10^-13 erg/s/cm2 at this position (from HIgh-energy LIght curve GeneraTor, HILIGT, Saxton et al. 2022).

Within the FXT error circle there are multiple optical sources of unclear classification, and a radio source VLASS1QLCIR J185208.77-192014.7 is also present. According to the Gaia DR3 catalog (Gaia Collaboration, 2023), no stars with distance < 1 kpc lie within the FXT error circle. The physical nature of this transient remains unclear. EP-FXT will continue monitoring and multi-wavelength observations are encouraged.

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).