Einstein Probe detection of an X-ray transient EP260507b

ATel #17783; D. Y. Li (NAO, CAS), S. Y. Fu (HUST), R. Shi (PMO, CAS), H. W. Pan (NAO, CAS) on behalf of the Einstein Probe team
on 9 May 2026; 09:37 UT
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)

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We report on the detection of an X-ray transient designated EP260507b, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient was first detected by WXT at 2026-05-07T19:00:27 (UTC). The WXT spectrum can be fitted with an absorbed powerlaw, with the absorption fixed at the Galactic value of 1.15 x 10^20 cm^-2, and a photon index of 0.4 (-0.8, +0.9). The unabsorbed 0.5-4 keV flux is 4.6 (-2.0, +2.6) x 10^-11 erg/s/cm2. We conducted a follow-up observation with the Follow-up X-ray telescope (FXT) on board EP which started at 2026-05-08T17:12:34 (UTC), about 22 hours after the WXT detection. The observation, with an exposure time of 2.4 ks, shows an uncatalogued X-ray source located at R.A. = 345.4380 deg, DEC = -48.1565 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). The Galactic coordinate of source is l = 340.9475 deg, b = -60.0770 deg. The average 0.5-10 keV spectrum can be fitted by an absorbed power law with a photon index of 1.62 (-0.10, +0.10), and an absorption column of 4.3 (-3.6, +3.8) x 10^20 cm^-2, giving an unabsorbed 0.5-10 keV flux of 2.64 (-0.15, +0.16) x 10^-11 erg/s/cm2. The uncertainties are at the 90% confidence level for the above parameters.

No known X-ray sources were found within the error circle of FXT position. The eROSITA all-sky survey in 2020 yielded a 0.2-5 keV flux upper limit of 1.4 x 10^-13 erg/s/cm2 (Tubín-Arenas et al. 2024, Merloni et al. 2024). More multi-wavelength follow-up observations are encouraged to explore the nature of this new source.

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). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.

Reference:

Merloni, A., et al., The SRG/eROSITA all-sky survey - First data release and X-ray catalog of the western Galactic hemisphere, A&A, 2024, 682, 34

Tubín-Arenas, D., et al., The eROSITA Upper Limits: Description and access to the data, A&A, 2024, 682, 35