GCN Circular 42377

C. Y. Dai (NJU), Q. J. Huang(PMO), W. F. Wen (SZTU), J. H. Wu (GZHU), H. W. Pan (NAO, CAS) on behalf of the Einstein Probe (EP) team:

We report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP251021b. The transient triggered EP-WXT (ID: 01709247263) at 2025-10-21T14:28:28 (UTC). The ground analysis of the WXT data shows that the event started at T0=2025-10-21T14:28:05 (UTC) and lasted for around 30 s. The WXT position of the source is R.A. = 306.742 deg, Dec. = -38.286 deg (J2000) with an uncertainty of 2.3 arcmin in radius (90% C.L. statistical and systematic). 

We performed a Target-of-Opportunity observation with the Follow-up X-ray Telescope on board EP. The observation began at 2025-10-22T04:41:25 (UTC), around 14 hours after the trigger. An uncatalogued X-ray source was detected within the WXT error circle at R.A. = 306.7451 deg, Dec. = –38.3048 deg (J2000), with a positional uncertainty of 20 arcsec in radius (90% confidence level, including both statistical and systematic uncertainties).

Further information will be updated when the telemetry data is received. 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).