EP260628c: Einstein Probe detection of an X-ray transient

ATel #17859; H. N. Yang, X. Mao, C. C. Jin (NAO, CAS), C. Y. Dai (NJU) on behalf of the Einstein Probe (EP) team
on 30 Jun 2026; 09:07 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 through ground data analysis, designated EP260628c. The transient was firstly detected in the WXT observation starting at 2026-06-28 16:22:49 (UTC). The WXT position of the source is R.A. = 339.500 deg, Dec = 53.226 deg (J2000) with an uncertainty of 2.7 arcmin in radius (90% C.L. statistical and systematic), located at low Galactic latitude (l = 103.648, b = -4.576). The 0.5–4 keV spectrum obtained in this observation can be fitted with an absorbed power-law model with the hydrogen column density of 7.6 (+1.5/-7.6) x 10^21 cm^-2 and photon index of 1.7 (+2.2/-1.6). The absorption-corrected 0.5-4 keV flux is 1.3 (+7.6/-0.6) x 10^-10 erg cm^-2 s^-1.

The Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) mission performed a follow-up observation on 2026-06-29 13:02:00 (UTC), with a net exposure of 2.5 ks. A bright uncatalogued X-ray source was detected within the WXT error circle at R.A. = 339.4980, and Dec. = 53.2394 (J2000), with an uncertainty radius of 10 arcseconds (90% confidence level, including both statistical and systematic uncertainties).

The FXT spectrum can be fitted with an absorbed power-law model. The best-fitting parameters are a hydrogen column density of 6.28 (+/-0.02) x 10^21 cm-2 and a photon index of 1.99 (+/-0.04). The absorption-corrected 0.5-10.0 keV flux is 1.61 (+/-0.03) x 10^-10 erg/s/cm^2. Using the barycenter-corrected light curve of this source, we can identify a significant periodic signal of approximately 3 seconds.

Further follow-up observations are encouraged to identify the nature of this 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).