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 1×10−13 to 1×10−11 erg cm−2 s−1 and T90 values from 0.3 to 12.1 ks. This sample of FXTs has likely redshifts between 0.7 to 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 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 ≳1×10−13 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 ∼50 %, and appear to have a similar diversity of possible progenitors.
@article{arxiv.2304.13795,
title = {Extragalactic FXT Candidates Discovered by Chandra (2014-2022)},
author = {J. Quirola-Vásquez and F. E. Bauer and P. G. Jonker and W. N. Brandt and G. Yang and A. J. Levan and Y. Q. Xue and D. Eappachen and E. Camacho and M. E. Ravasio and X. C. Zheng and B. Luo},
journal= {arXiv preprint arXiv:2304.13795},
year = {2023}
}
Comments
37 pages, 19 figures, 11 tables, accepted for publication in A&A. arXiv admin note: text overlap with arXiv:2201.07773