EP260530a: Einstein Probe detection of an X-ray transient

ATel #17824; Q. Y. Wu (NAO, CAS), C. Y. Dai (NJU), W. F. Wen (SZTU), H. Sun (NAO, CAS) on behalf of the Einstein Probe (EP) team
on 31 May 2026; 09:18 UT
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)

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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 2026-05-30T07:29:02 (UTC), designated EP260530a. The position of the source is R.A. = 186.110 deg, Dec. = 28.784 deg, with an uncertainty of 2.7 arcmin in radius (90% C.L. statistical and systematic). Analysis of this WXT observation shows that its spectrum can be fitted with a blackbody model, with the absorption column density fixed at the Galactic value of 2.16 x 10^20 cm^-2. The best-fit temperature is 63 (-11/+13) eV, and the unabsorbed flux in the 0.5-4 keV band is 1.5 (-0.3/+0.3) x 10^-11 erg/s/cm^2. No significant variability was detected in the light curve in one WXT observation. The transient was subsequently detected in two WXT observations at 2026-05-30T12:16:31 and 2026-05-30T17:03:24, with the flux inferred from the latest observation of 1.2 (-0.4/+0.5) x 10^-11 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

A follow-up observation with the Follow-up X-ray Telescope (FXT) on-board EP was performed around 2026-05-31T05:53:29 (UTC). Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 186.1022 deg, DEC = 28.7883 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic).

We note that EP260530a is positionally (~9 arcsec) and temporally consistent with the optical transient AT 2026nwl reported to the IAU Transient Name Server ( https://www.wis-tns.org/object/2026nwl ).

Further information will be updated when the telemetry data is received. Multi-band follow-up observations are encouraged to identify the nature of this 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).