J/A+A/675/A44 Extragalactic fast X-ray transient cand. (Quirola-Vasquez+, 2023)
Extragalactic fast X-ray transient candidates discovered by Chandra (2014-2022).
Quirola-Vasquez J., Bauer F.E., Jonker P.G., Brandt W.N., Yang G.,
Levan A.J., Xue Y.Q., Eappachen D., Camacho E., Ravasio M.E., Zheng X.C.,
Luo B.
<Astron. Astrophys. 675, A44 (2023)>
=2023A&A...675A..44Q 2023A&A...675A..44Q (SIMBAD/NED BibCode)
ADC_Keywords: X-ray sources
Keywords: X-ray: bursts
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,
using data taken between 2000-2014) 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 (Quirola et al.,
2022A&A...663A.168Q 2022A&A...663A.168Q).
We report the detection of eight FXT candidates, with peak 0.3-10keV
fluxes between 1*10-13 to 1*10-11erg/cm2/s and T90 values from
0.3 to 12.1ks. This sample of FXTs has likely redshifts between 0.7 to
1.8. Three FXT candidates exhibit light curves with a plateau (∼1-3ks
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+9:7-8.3deg2*yr-1 for
the extragalactic samples for a limiting flux of
≳1*10-13erg/cm2/s, 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.
Description:
In this work we searched for extragalactic FXTs present in Chandra
data from 2014 to 2022.
We applied an algorithm developed by Yang et al. (2019MNRAS.487.4721Y 2019MNRAS.487.4721Y)
and Quirola-Vasquez et al. (2022A&A...663A.168Q 2022A&A...663A.168Q, hereafter Paper I) to
X-ray sources with |b|>10deg (i.e., 3899 Chandra observations,
totaling 88.8Ms and 264.4deg2). Considering additional criteria
(analyzing further X-ray observations taken by Chandra, XMM-Newton,
Swift-XRT, Einstein, and ROSAT) and other astronomical catalogs (e.g.,
Gaia, NED, SIMBAD, VHS, DES, Pan-STARRS), we identify eight FXTs
consistent with an extragalactic origin. We rediscover all (three)
previously reported Chandra sources: XRT 150322 (previously identified
by Xue et al., 2019Natur.568..198X 2019Natur.568..198X), XRT 170901 (previously identified
by Lin et al., 2019ATel13171....1L 2019ATel13171....1L, 2022ApJ...927..211L 2022ApJ...927..211L), and XRT
210423 (previously identified by Lin et al., 2021ATel14599....1L 2021ATel14599....1L).
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table3.dat 101 8 Properties of the extragalactic FXT candidates
detected and/or discussed in this work,
ordered by date.
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Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 2 I2 --- FXT [15/22] Shorthand identifier (FXT NN)
used throughout this work
4- 13 A10 --- XRT X-ray transient identifier (XRT date),
plus previous name when available
15 A1 --- n_XRT [+] Note on XRT (1)
17- 21 I5 --- ObsID Chandra observation ID
23- 26 F4.1 ks Texp Chandra observation exposure time
28- 43 A16 --- Date Chandra observation date
44- 47 F4.1 ks T90 T90 duration, which measures the time over
which the source emits the central 90%
(i.e., from % to 95%) of its total measured
counts, in units of ks
49- 51 F3.1 ks e_T90 Error on T90 (lower limit)
53- 55 F3.1 ks E_T90 Error on T90 (upper limit)
57- 65 F9.5 deg RAdeg Right Ascension (J2000)
67- 75 F9.5 deg DEdeg Declination (J2000)
77- 80 F4.1 arcmin OffAxis Off-axis angle of the FXT candidates, with
respect to the Chandra aimpoint
82- 85 F4.2 arcsec ePos Estimated 2σ X-ray positional
uncertainty (see Sect. 2.5).
87- 91 F5.2 --- HR Hardness ratio (2)
93- 96 F4.2 --- e_HR Hardness ratio 1σ uncertainty
98-101 F4.1 --- S/N Approximate signal-to-noise ratio (S/N)
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Note (1): Note as follows:
+ = Previously reported as FXTs
by Xue et al. (2019Natur.568..198X 2019Natur.568..198X) for FXT 16 ([BTS2017] CDF-S XT2),
Lin et al. (2019ATel13171....1L 2019ATel13171....1L) for FXT 19, and
Lin et al. (2021ATel14599....1L 2021ATel14599....1L) for FXT 22
Note (2): Hardness ratio defined as HR=(H-S)/(H+S) where H=2-7keV and
S=0.5-2keV energy bands, using the Bayesian estimation of Park et al.
(2006ApJ...652..610P 2006ApJ...652..610P).
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Acknowledgements:
Jonathan Quirola-Vasquez, jaquirola(at)uc.cl
References:
Quirola et al., Paper I 2022A&A...663A.168Q 2022A&A...663A.168Q
(End) Patricia Vannier [CDS] 15-May-2023