Extragalactic fast X-ray transient candidates discovered by Chandra (2014–2022)^⋆

¹ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
e-mail: jaquirola@uc.cl
² Millennium Institute of Astrophysics (MAS), Nuncio Monseñor Sótero Sanz 100, Providencia, Santiago, Chile
³ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010 6500 GL Nijmegen, The Netherlands
⁴ Observatorio Astronómico de Quito, Escuela Politécnica Nacional, 170136 Quito, Ecuador
⁵ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado, 80301 USA
⁶ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
⁷ Department of Astronomy & Astrophysics, 525 Davey Laboratory, The Pennsylvania State University, University Park, PA, 16802 USA
⁸ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA, 16802 USA
⁹ Department of Physics, 104 Davey Laboratory, The Pennsylvania State University, University Park, PA, 16802 USA
¹⁰ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
¹¹ SRON Netherlands Institute for Space Research, Postbus 800, 9700 AV Groningen, The Netherlands
¹² Department of Physics, University of Warwick, Coventry, CV4 7AL UK
¹³ CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei, 230026 PR China
¹⁴ School of Astronomy and Space Science, University of Science and Technology of China, Hefei, 230026 PR China
¹⁵ INAF – Brera Astronomical Observatory, Via Bianchi 46, 23807 Merate, LC, Italy
¹⁶ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
¹⁷ School of Astronomy and Space Science, Nanjing University, Nanjing, PR China
¹⁸ Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing, 210093 PR China

Received: 13 January 2023
Accepted: 26 April 2023

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.