By G. Yang

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⁻¹deg⁻² for CDF-S XT-like transients with 0.5-7 keV peak fluxes logF_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 × field of view) are needed to discover a large sample (∼ thousands) of fast extragalactic X-ray transients.

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