Extragalactic FXT Candidates Discovered by Chandra (2014-2022)

Abstract

Extragalactic fast X-ray transients (FXTs) are short flashes of X-ray photons of unknown origin that last a few minutes to hours. We extend the search for extragalactic FXTs from Quirola et al. 2022 (Paper I; based on sources in the Chandra Source Catalog 2.0, CSC2) to further Chandra archival data between 2014-2022. We extract X-ray data using a method similar to that employed by CSC2 and apply identical search criteria as in Paper I. We report the detection of eight FXT candidates, with peak 0.3-10 keV fluxes between 1$\times$10${}^{-13}$ to 1$\times$10${}^{-11}$ erg cm${}^{-2}$ s${}^{-1}$ and $T_{90}$ values from 0.3 to 12.1 ks. This sample of FXTs has likely redshifts between 0.7 to 1.8. Three FXT candidates exhibit light curves with a plateau ($\approx$1-3 ks duration) followed by a power-law decay and X-ray spectral softening, similar to what was observed for a few previously reported FXTs in Paper I. In light of the new, expanded source lists (eight FXTs with known redshifts from Paper I and this work), we update the event sky rates derived in Paper I, finding 36.9${}_{-8.3}^{+9.7}$ deg${}^{-2}$ yr${}^{-1}$ for the extragalactic samples for a limiting flux of $\gtrsim$1$\times$10${}^{-13}$ erg cm${}^{-2}$ s${}^{-1}$, calculate the first FXT X-ray luminosity function, and compare the volumetric density rate between FXTs and other transient classes. Our latest Chandra-detected extragalactic FXT candidates boost the total Chandra sample by $\sim$50 %, and appear to have a similar diversity of possible progenitors.

37 pages, 19 figures, 11 tables, accepted for publication in A&A. arXiv admin note: text overlap with arXiv:2201.07773