2003/05/22 by C. O. Heinke, J. E. Grindlay, P. M. Lugger +4 · 4 citations
Physics and Astronomy · #Astrophysical Phenomena and Observations #Pulsars and Gravitational Waves Research #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/378885
published as Astrophys.J. 598 (2003) 501-515 · Submitted to ApJ. 22 pages, seven figures
arxiv created 2003/05/22 · openalex publication_date 2003/11/20 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/30
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 10 31 ergs s -1 < L X < 10 32.5 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 ~10 32 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.