J/MNRAS/479/2834 233 X-ray sources in omega Centauri (Henleywillis+, 2018)
A deep X-ray Survey of the globular cluster omega Centauri.
Henleywillis S., Cool A.M., Haggard D., Heinke C., Callanan P., Zhao Y.
<Mon. Not. R. Astron. Soc., 479, 2834-2852 (2018)>
=2018MNRAS.479.2834H 2018MNRAS.479.2834H (SIMBAD/NED BibCode)
ADC_Keywords: Clusters, globular ; X-ray sources
Keywords: binaries: close - novae - cataclysmic variables -
globular clusters: individual: omega Centauri - NGC 5139 -
X-rays: binaries
Abstract:
We identify 233 X-ray sources, of which 95 are new, in a 222ks
exposure of omega Centauri with the Chandra X-ray Observatory's
Advanced CCD Imaging Spectrometer detector. The limiting unabsorbed
flux in the core is fX(0.5-6.0keV)~=3x10-16erg/s/cm2
(Lx~=1x1030erg/s at 5.2kpc). We estimate that ∼60±20 of these are
cluster members, of which ∼30 lie within the core (rc=155-arcsec),
and another ∼30 between 1-2 core radii. We identify four new optical
counterparts, for a total of 45 likely identifications. Probable
cluster members include 18 cataclysmic variables (CVs) and CV
candidates, one quiescent low-mass X-ray binary, four variable stars,
and five stars that are either associated with omega Cen's anomalous
red giant branch or are sub-subgiants. We estimate that the cluster
contains 40±10 CVs with Lx>1031erg/s, confirming that CVs are
underabundant in omega Cen relative to the field. Intrinsic absorption
is required to fit X-ray spectra of six of the nine brightest CVs,
suggesting magnetic CVs, or high-inclination systems. Though no radio
millisecond pulsars (MSPs) are currently known in omega Cen, more than
30 unidentified sources have luminosities and X-ray colours like those
of MSPs found in other globular clusters; these could be responsible
for the Fermi-detected gamma-ray emission from the cluster. Finally,
we identify a CH star as the counterpart to the second brightest X-ray
source in the cluster and argue that it is a symbiotic star. This is
the first such giant/white dwarf binary to be identified in a globular
cluster.
Description:
We obtained two long exposures of omega Cen using the imaging array of
the Chandra X-ray Observatory's ACIS-I on 2012 April 16 and 17. The
data sets have a combined exposure time of ∼222ks (173.7 and 48.5ks
for ObsIDs 13726 and 13727, respectively).
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 115 233 Full set of 233 sources that appear in the
2012 data set, 95 of which are new
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See also:
J/ApJ/697/224 : Chandra X-ray sources in ω Cen (Haggard+, 2009)
J/A+A/493/959 : UBV(RI)cHalpha photometry in omega Cen (Bellini+, 2009)
J/ApJ/710/1032 : Proper motions in ω Cen (Anderson+, 2010)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 4 A4 --- Name Name from Haggard et al. (2009,
Cat. J/ApJ/697/224, [HCD2009] NNNa in
Simbad) or this work
5 A1 --- n_Name [*] * for names assigned in this work
7 A1 --- --- [J]
8- 9 I2 h RAh Right ascension (J2000)
10- 11 I2 min RAm Right ascension (J2000)
12- 17 F6.3 s RAs Right ascension (J2000)
18 A1 --- DE- Declination sign (J2000)
19- 20 I2 deg DEd Declination (J2000)
21- 22 I2 arcmin DEm Declination (J2000)
23- 27 F5.2 arcsec DEs Declination (J2000)
29- 32 F4.2 arcsec ePos 95% confidence error circle radius
calculated using equation 5 of Hong et al.
(2005ApJ...635..907H 2005ApJ...635..907H)
34- 37 F4.2 --- Offset Offset (1)
39- 42 I4 ct XMd Detected medium (0.5-4.5keV) band X-ray
counts
43 A1 --- --- [/]
44- 49 F6.1 ct XMc Corrected medium (0.5-4.5keV) band X-ray
counts
51- 53 I3 ct XSd Detected soft (0.5-1.5keV) band X-ray counts
54 A1 --- --- [/]
55- 60 F6.1 ct XSc Corrected soft (0.5-1.5keV) band X-ray
counts
62- 65 I4 ct XHd Detected hard (1.5-6.0keV) band X-ray counts
66 A1 --- --- [/]
67- 72 F6.1 ct XHc Corrected hard (1.5-6.0keV) band X-ray
counts
74- 78 F5.2 [-] log(XS/XH) ?=- Soft-to-hard X-ray count ratio
80- 85 F6.1 10-19W/m2 FXM Medium (0.5-4.5keV) band X-ray flux
87- 92 F6.1 10-19W/m2 FXT Total (0.5-6.0keV) band X-ray flux
94- 99 F6.1 10-19W/m2 FXS Soft ((0.5-2.0keV) band X-ray flux
101-104 F4.2 --- FXratio ?=- 2012/2000 0.5-6.0keV band flux ratio (2)
105 A1 --- n_FXratio [g] Note on FXratio (3)
107-115 A9 --- OID Optical ID (4)
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Note (1): Offset from Anderson & van der Marel (2010, Cat. J/ApJ/710/1032)
cluster centre (RA=13 26 47.24, DE=-47 28 46.45) in units of the
core radius (rc=155").
Note (2): Unabsorbed fluxes determined assuming a power-law spectrum with
photon index GAMMA=1.4.
Note (3): g: Fluxes determined using a spectrum appropriate for this object show
no change from 2000 to 2012 (Heinke et al., 2014MNRAS.444..443H 2014MNRAS.444..443H).
Note (4): Optical identifications from Haggard et al. (2009,
Cat. J/ApJ/697/224) Cool et al. (2013ApJ...763..126C 2013ApJ...763..126C), and this work;
'NV' and 'V' IDs are variable stars from Kaluzny et al. (2004,
Cat. J/A+A/424/1101);
parentheses indicate that stars are non-members according to Bellini et al.
(2009, Cat. J/A+A/493/959).
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History:
From electronic version of the journal
(End) Patricia Vannier [CDS] 05-Mar-2019