2019MNRAS.487.4721Y

Recent works have discovered two fast (≃10 ks) extragalactic X-ray transients in the Chandra Deep Field-South (CDF-S XT1 and XT2). These findings suggest that a large population of similar extragalactic transients might exist in archival X-ray observations. We develop a method that can effectively detect such transients in a single Chandra exposure, and systematically apply it to Chandra surveys of CDF-S, CDF-N, DEEP2, UDS, COSMOS, and E-CDF-S, totaling 19 Ms of exposure. We find 13 transient candidates, including CDF-S XT1 and XT2. With the aid of available excellent multiwavelength observations, we identify the physical nature of all these candidates. Aside from CDF-S XT1 and XT2, the other 11 sources are all stellar objects, and all of them have z-band magnitudes brighter than 20. We estimate an event rate of 59⁺⁷⁷_–38 evt yr^–1 deg^–2 for CDF-S XT-like transients with 0.5-7 keV peak fluxes log F_peak >= -12.6 (erg cm^–2 s^–1). This event rate translates to ~15⁺²⁰_–10 transients existing among Chandra archival observations at Galactic latitudes |b| > 20°, which can be probed in future work. Future missions such as Athena and the Einstein Probe with large grasps (effective area x field of view) are needed to discover a large sample (∼thousands) of fast extragalactic X-ray transients.