2023A&A...675A..44Q

Context. Extragalactic fast X-ray transients (FXTs) are short flashes of X-ray photons of unknown origin that last a few minutes to hours.
Aims. We extend the previous search for extragalactic FXTs (based on sources in the Chandra Source Catalog 2.0, CSC2) to further Chandra archival data between 2014 and 2022.
Methods. We extracted X-ray data using a method similar to that employed by CSC2 and applied identical search criteria as in previous work.
Results. We report the detection of eight FXT candidates, with peak 0.3-10 keV fluxes between 1 × 10^–13 to 1 × 10^–11 erg cm^–2 s^–1 and T₉₀ values from 0.3 to 12.1 ks. This sample of FXTs likely has redshifts between 0.7 and 1.8. Three FXT candidates exhibit light curves with a plateau (≈1-3 ks duration) followed by a power-law decay and X-ray spectral softening, similar to what was observed for a few before-reported FXTs. In light of the new, expanded source lists (eight FXTs with known redshifts from a previous paper and this work), we have updated the event sky rates derived previously, finding 36.9$ _{-8.3}^{+9.7} $ deg^–2 yr^–1 for the extragalactic samples for a limiting flux of ≳1 × 10^–13 erg cm^–2 s^–1, calculated the first FXT X-ray luminosity function, and compared the volumetric density rate between FXTs and other transient classes.
Conclusions. Our latest Chandra-detected extragalactic FXT candidates boost the total Chandra sample by ∼50%, and appear to have a similar diversity of possible progenitors.