Analysis of the Quiescent Low-Mass X-Ray Binary Population in Galactic Globular Clusters
By C. Heinke, J. Grindlay, P. Lugger, H. Cohn, P. Edmonds, D. Lloyd, A. Cool. 2003
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Quiescent low-mass X-ray binaries (qLMXBs) containing neutron stars have been identified in several globular clusters using Chandra or XMM X-ray observations, via their distinctive soft thermal spectra. We report a complete census of the qLMXB population in these clusters, identifying three additional probable qLMXBs in NGC 6440. We conduct several analyses of the qLMXB population and compare it with the harder, primarily cataclysmic variable (CV), population of low-luminosity X-ray sources with 1031 ergs s-1 < LX < 1032.5 ergs s-1. The radial distribution of our qLMXB sample suggests an average system mass of 1.5 M☉, consistent with a neutron star and low-mass companion. Spectral analysis reveals that no globular cluster qLMXBs, other than the transient in NGC 6440, require an additional hard power-law component, as often observed in field qLMXBs. We identify an empirical lower luminosity limit of ~1032 ergs s-1 among globular cluster qLMXBs. The bolometric luminosity range of qLMXBs implies (in the deep crustal heating model of Brown and collaborators) low time-averaged mass-transfer rates, below the disk stability criterion. The X-ray luminosity functions of the CV populations alone in NGC 6397 and 47 Tuc are shown to differ. The distribution of qLMXBs among globular clusters is consistent with their dynamical formation by either tidal capture or exchange encounters, allowing us to estimate that 7 times more qLMXBs than bright LMXBs reside in globular clusters. The distribution of harder sources (primarily CVs) has a weaker dependence on density than that of the qLMXBs. Finally, we discuss possible effects of core collapse and globular cluster destruction on X-ray source populations.
Subfields: Astronomy and Astrophysics
View Paper: https://www.semanticscholar.org/paper/46ea12db8c49907205caf3a4471e011d46a91ebb
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In Astronomy and Astrophysics
Citations in the corpus
- Formation and evolution of compact binaries in globular clusters – II. Binaries with neutron stars (2007, methodology): “5 of them, 47 Tuc, Terzan 5, NGC 6266, NGC 6388 and NGC 6440 are known to contain 5 or more qLMXBs (Heinke et al. 2003, Cohn et al. 2008 in prep, ).”
- A Deep Chandra Survey of the Globular Cluster 47 Tucanae: Catalog of Point Sources (2005, unknown): “The two previously identified qLMXBs in 47 Tuc display the same spectral and variability characteristics as in the 2000 data set (Heinke et al. 2003b).”
- Bayesian Estimation of Hardness Ratios: Modeling and Computations (2006, background): “Although only the identification of their optical counterparts can unambiguously classify an X-ray source as a qLMXB, soft X-ray spectra or hardness ratios consistent with those expected for thermal emission by neutron stars are very useful for identifying candidate qLMXB’s (Heinke et al. 2003).”
- Constraining the mass and radius of neutron stars in globular clusters (2017, background): “Finally, unlike the majority of quiescent LMXBs found outside globular clusters (through recent outbursts), quiescent LMXBs identified in globular clusters tend to have relatively simple spectra, dominated by thermal surface emission with little or no powerlaw component (Heinke et al. 2003b).”