Extragalactic fast X-ray transient candidates discovered by Chandra (2014-2022)
Bauer, F. E.; Brandt, W. N.; Xue, Y. Q.; Luo, B.; Quirola-Vásquez, J.; Yang, G.; Zheng, X. C.; Levan, A. J.; Ravasio, M. E.; Jonker, P. G.; Eappachen, D.; Camacho, E.
Chile, Netherlands, Ecuador, United States, United Kingdom, China, Italy
Abstract
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.