Searching for fast extragalactic X-ray transients in Chandra surveys.

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 |${59^{+77}_{-38}\ \rm {evt\ yr^{-1}\, deg^{-2}}}$| 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 |${\approx 15^{+20}_{-10}}$| 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. [ABSTRACT FROM AUTHOR]