The X-ray Emissivity of Low-Density Stellar Populations

Abstract

The dynamical production of low-mass X-ray binaries and brighter cataclysmic variables (CVs) in dense globular clusters is well-established. We investigate how the X-ray emissivity of fainter X-ray binaries (principally CVs and coronally active binaries) varies between different environments. We compile calculations (largely from the literature) of the X-ray emissivity of old stellar populations, including open and globular clusters and several galaxies. We investigate three literature claims of unusual X-ray sources in low-density stellar populations. We show that a suggested quiescent neutron star in the open cluster NGC 6819 is a foreground M dwarf. We show that the suggested diffuse X-ray emission from an old nova shell in the globular cluster NGC 6366 is actually a background galaxy cluster. And we show that a suggested population of quiescent X-ray binaries in the Sculptor Dwarf Galaxy is mostly (perhaps entirely) background galaxies. We find that above densities of $10^4$ M${}_{\odot }$/pc${}^3$, the X-ray emissivity of globular clusters increases, due to dynamical production of X-ray emitting systems. Below this density, globular clusters have lower X-ray emissivity than the other populations, and we do not see a strong dependence of X-ray emissivity due to density effects. We find significant correlations between X-ray emissivity and binary fraction, metallicity, and density. Sampling these fits via bootstrap techniques gives less significant correlations, but confirms the effect of metallicity on low-density populations, and that of density on the full globular cluster sample.

28 pages, 23 figures. MNRAS, in press

References in corpus (46)

• A catalogue of low-mass X-ray binaries in the Galaxy, LMC, and SMC (Fourth edition)
• Near-IR properties of 24 Globular Clusters in the Galactic Bulge
• The fraction of binary systems in the core of thirteen low-density Galactic globular clusters
• A Binary Origin for Blue Stragglers in Globular Clusters
• Formation and evolution of compact binaries in globular clusters: I.Binaries with white dwarfs
• Chandra X-Ray Observations of Nineteen Millisecond Pulsars in the Globular Cluster 47 Tucanae
• Dynamical Formation of Close Binaries in Globular Clusters II: Cataclysmic Variables
• Populating the Galaxy with low-mass X-ray binaries
• The core binary fractions of star clusters from realistic simulations
• The Araucaria Project. The Distance to the Sculptor dwarf spheroidal galaxy from infrared photometry of RR Lyrae stars
• Faint X-ray Sources in the Globular Cluster Terzan 5
• Universal X-ray emissivity of the stellar population in early-type galaxies: unresolved X-ray sources in NGC 3379
• WIYN Open Cluster Study. LX. Spectroscopic Binary Orbits in NGC 6819
• A new standard: Age and distance for the open cluster NGC 6791 from the eclipsing binary member V20
• The Puzzling White Dwarf Cooling Sequence in NGC6791: A Simple Solution
• Comparing GC and Field LMXBs in Elliptical Galaxies with deep Chandra and Hubble data
• The space density of cataclysmic variables: constraints from the ROSAT North Ecliptic Pole Survey
• A New Look at the Old Star Cluster NGC 6791
• The state of globular clusters at birth II: primordial binaries
• Identification of Faint Chandra X-ray Sources in the Core-Collapsed Globular Cluster NGC 6397: Evidence for a Bimodal Cataclysmic Variable Population
• X-Ray-Emitting Stars Identified from the ROSAT All-Sky Survey and the Sloan Digital Sky Survey
• The end of the white dwarf cooling sequence in M 67
• X-ray emission from the stellar population in M32
• The evolution of the binary population in globular clusters: a full analytical computation
• Chandra X-ray Sources in the Collapsed-Core Globular Cluster M30 (NGC 7099)
• A search for spectroscopic binaries in the galactic globular cluster M4. Based on 5973 individual spectra collected at VLT
• The Parkes multibeam pulsar survey: VII. Timing of four millisecond pulsars and the underlying spin period distribution of the Galactic millisecond pulsar population
• X-ray and optical observations of M55 and NGC 6366: evidence for primordial binaries
• Spectroscopic Abundances in the Open Cluster, NGC 6819
• Variable stars in the open cluster NGC 6791 and its surrounding field
• Discovery of Hot Gas in Outflow in NGC3379
• Chandra X-Ray Observatory Observations of the Globular Cluster M71
• Dynamical Formation of Cataclysmic Variables in Globular Clusters
• X-ray Spectral Identification of Three Candidate Quiescent Low-Mass X-ray Binaries in the Globular Cluster NGC 6304
• XMM-Newton observations of the Galactic globular clusters NGC 2808 and NGC 4372
• Low-luminosity X-ray sources and the Galactic ridge X-ray emission
• Combined Chandra, XMM-Newton and Hubble Space Telescope observations of the Galactic globular cluster NGC 2808
• Diffuse X-ray emission in globular cluster cores
• Chandra Observation of Quiescent Low-Mass X-ray Binaries in the Globular Cluster NGC 6304
• Ages of Elliptical Galaxies: Single versus Multi Population Interpretation
• Observational evidence for the origin of X-ray sources in globular clusters
• Variability of Optical Counterparts in the Chandra Galactic Bulge Survey
• Chandra detection of a new diffuse X-ray component from the globular cluster 47 Tucanae
• A Chandra X-ray census of the interacting binaries in old open clusters - Collinder 261
• A New Pulsar in Green Bank Telescope Searches of Ten Globular Clusters
• Multiwavelength survey of X-ray sources in the Sculptor Dwarf Spheroidal Galaxy

Cited by in corpus (3)

• The MAVERIC Survey: Chandra/ACIS Catalog of Faint X-ray sources in 38 Galactic globular clusters
• A Deep Search for Faint Chandra X-ray Sources, Radio Sources, and Optical Counterparts in NGC 6752
• Periodic X-ray sources in the Massive Globular Cluster 47 Tucanae: Evidence for Dynamically Formed Cataclysmic Variables