Extragalactic fast X-ray transient candidates discovered by Chandra (2000–2014)

¹ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
e-mail: jquirola@astro.puc.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 Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301, USA
⁵ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
⁶ Department of Astronomy & Astrophysics, 525 Davey Laboratory, The Pennsylvania State University, University Park, PA 16802, USA
⁷ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
⁸ Department of Physics, 104 Davey Laboratory, The Pennsylvania State University, University Park, PA 16802, USA
⁹ Texas A&M University, Physics and Astronomy, 4242 TAMU, College Station, TX 77843-4242, USA
¹⁰ George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University, College Station, TX 77843-4242, USA
¹¹ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
¹² CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei 230026, PR China
¹³ School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, PR China
¹⁴ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
¹⁵ School of Astronomy and Space Science, Nanjing University, Nanjing, PR China
¹⁶ Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210093, PR China

Received: 5 January 2022
Accepted: 11 April 2022

Abstract

Context. Extragalactic fast X-ray transients (FXRTs) are short flashes of X-ray photons of unknown origin that last a few seconds to hours.

Aims. Our ignorance about their physical mechanisms and progenitor systems is due in part to the lack of clear multiwavelength counterparts in most cases, because FXRTs have only been identified serendipitously.

Methods. We develop a systematic search for FXRTs in the Chandra Source Catalog (Data Release 2.0; 169.6 Ms over 592.4 deg², using only observations with |b|> 10° and before 2015), using a straightforward X-ray flare search algorithm and incorporating various multiwavelength constraints to rule out Galactic contamination and characterize the candidates.

Results. We report the detection of 14 FXRT candidates from a parent sample of 214 701 sources. Candidates have peak 0.5–7 keV fluxes between 1 × 10⁻¹³ and 2 × 10⁻¹⁰ erg cm⁻² s⁻¹ and T₉₀ values from 4 to 48 ks. The sample can be subdivided into two groups: six “nearby” FXRTs that occurred within d  ≲  100 Mpc and eight “distant” FXRTs with likely redshifts ≳0.1. Three distant FXRT 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 the previously reported FXRT CDF-S XT2, a proposed magnetar-powered binary neutron star merger event. After applying completeness corrections, we calculate event rates for the nearby and distant samples of 53.7_−15.1^+22.6 and 28.2_−6.9^+9.8 deg⁻² yr⁻¹, respectively.

Conclusions. This novel sample of Chandra-detected extragalactic FXRT candidates, although modest in size, breaks new ground in terms of characterizing the diverse properties, nature, and possible progenitors of these enigmatic events.