GCN Circular 45779

C. L. Guo, B. B. Zhang, K. Ao, S. Q. Jiang, H. Sun (NAO, CAS), D. F. Hu (PMO, CAS), A. D. Zhu(HUST), J. Y. Liu(PMO,CAS), on behalf of the Einstein Probe (EP) team: 

We report the discovery of a new fast X-ray transient EP260927a, identified in the EP-WXT telemetry data at RA = 31.833 deg, Dec =-27.169 deg (J2000), with an uncertainty of 2.6 arcmin in radius (90%, C.L statistical and systematic). The transient started about 2026-09-27T14:02:35 (UTC) and lasted for about 800 s. The 0.5-4 keV flux estimated from the WXT data is approximately 2.91x10^-11 erg/s/cm^2. We note the presence of a historical X-ray source 1eRASS J020729.2-271052, with flux density 7.79e-13 erg/s/cm^2, located 2.2 arcmin away from the center of the WXT error circle.This source is likely associated with Gaia DR3 5117556925932530176 and may be a QSO candidate according to GAIA classification.

We performed a follow-up observation of EP260927a with the Follow-up X-ray Telescope on board EP to confirm the transient nature of the WXT detection. The observation started at 2026-09-28T03:53:50 (UTC). The effective exposure time is around 6 ks. On-ground analysis of the FXT data found two sources within the WXT error circle. The first source is an uncatalogued source, detected at RA = 31.8105 deg, Dec =-27.1855 deg (J2000), with an uncertainty of 10 arcsec in radius (90%, C.L statistical and systematic). The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 1.47 × 10^20 cm^-2, and a photon index of 2.28(-0.78/+0.87). The derived average unabsorbed 0.5-10 keV flux is 5.6(+4.9/-2.0)× 10^(-14) erg/s/cm^2. 

The position of the second source is consistent with the historical X-ray source 1eRASS J020729.2-271052. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 1.47 × 10^20 cm^-2, and a photon index of 2.7(-0.3/+0.3). The derived average unabsorbed 0.5-10 keV flux is 2.5(+0.4/-0.4)× 10^(-13) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above spectral parameters.

We tend to suggest that the first FXT source is the X-ray counterpart of EP260927a. Multi-wavelength follow-up 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).