Analysis of the Quiescent Low-Mass X-Ray Binary Population in Galactic Globular Clusters

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.

[1]  L. Bildsten,et al.  Crustal Heating and Quiescent Emission from Transiently Accreting Neutron Stars , 1998, astro-ph/9807179.

[2]  Optical Detection of a Variable Millisecond Pulsar Companion in 47 Tucanae , 2001, astro-ph/0107096.

[3]  J. Katz,et al.  Two kinds of stellar collapse , 1975, Nature.

[4]  S. Anderson,et al.  Optical Identification of Multiple Faint X-Ray Sources in the Globular Cluster NGC 6752: Evidence for Numerous Cataclysmic Variables , 2001, astro-ph/0110192.

[5]  Terrence M. Girard,et al.  Space Velocities of Globular Clusters. III. Cluster Orbits and Halo Substructure , 1999 .

[6]  D. McCammon,et al.  Photoelectric absorption cross sections with variable abundances , 1992 .

[7]  F. A. Rasio,et al.  Monte Carlo Simulations of Globular Cluster Evolution. III. Primordial Binary Interactions , 2003, astro-ph/0301521.

[8]  V. M. Kaspi,et al.  Dynamical Formation of Close Binary Systems in Globular Clusters , 2003 .

[9]  S. Campana,et al.  The neutron stars of Soft X–ray Transients , 1993, astro-ph/9805079.

[10]  M. Perrin,et al.  Metallicity and radial velocity of the second parameter globular cluster Terzan 1 in the galactic bulge , 2002 .

[11]  F. Camilo,et al.  Timing of Millisecond Pulsars in NGC 6752: Evidence for a High Mass-to-Light Ratio in the Cluster Core , 2002, astro-ph/0204283.

[12]  T. Maccarone,et al.  The Low-Mass X-Ray Binary-Globular Cluster Connection in NGC 4472 , 2002, astro-ph/0206221.

[13]  Philipp Podsiadlowski,et al.  Submitted to ApJ Preprint typeset using L ATEX style emulateapj v. 21/08/00 A COMPREHENSIVE STUDY OF NEUTRON STAR RETENTION IN GLOBULAR CLUSTERS , 2001 .

[14]  J. Mathis,et al.  The relationship between infrared, optical, and ultraviolet extinction , 1989 .

[15]  R. Gilliland,et al.  A Millisecond Pulsar Optical Counterpart with Large-Amplitude Variability in the Globular Cluster 47 Tucanae , 2002, astro-ph/0207426.

[16]  Simon F. Portegies Zwart,et al.  The Evolution of Globular Clusters in the Galaxy , 1999, astro-ph/9903366.

[17]  J. E. Grindlay,et al.  A Chandra X-Ray Study of the Dense Globular Cluster Terzan 5 , 2003, astro-ph/0303141.

[18]  F. Paresce,et al.  VLT Observations of the Peculiar Globular Cluster NGC 6712. III. The Evolved Stellar Population , 2001, astro-ph/0102331.

[19]  J. Grindlay.  X-raying the dynamics of globular clusters , 1985 .

[20]  Denmark,et al.  Distances and ages of NGC 6397, NGC 6752 and 47 Tuc , 2003, astro-ph/0307016.

[21]  Deep Hubble Space Telescope WFPC2 Photometry of NGC 288. II. The Main-Sequence Luminosity Function* , 2002, astro-ph/0202176.

[22]  J. Grindlay,et al.  CCD photometry of globular cluster core structure. 2: U-band profiles for 15 candidate collapsed-core clusters , 1995 .

[23]  B. Anthony-Twarog,et al.  CCD uvbyHβ Photometry in Clusters. II. The Nearest Globular Cluster, NGC 6397 , 2000 .

[24]  Toulouse,et al.  Discovery of a quiescent neutron star binary in the globular cluster M 13 , 2003, astro-ph/0303471.

[25]  Seattle,et al.  Chandra Observation of the Globular Cluster NGC 6440 and the Nature of Cluster X-Ray Luminosity Functions , 2001, astro-ph/0111212.

[26]  S. Murray,et al.  Determination of the mass of globular cluster X-ray sources , 1984 .

[27]  M. Davies.  Cataclysmic variable production in globular clusters , 1996 .

[28]  N. Gehrels.  Confidence limits for small numbers of events in astrophysical data , 1986 .

[29]  Robert E. Rutledge,et al.  A Possible Transient Neutron Star in Quiescence in the Globular Cluster NGC 5139 , 2001 .

[30]  B. Gendre,et al.  First XMM-Newton observations of the globular cluster M 22 , 2001, astro-ph/0110657.

[31]  R. Wijnands,et al.  A Chandra Observation of the Globular Cluster Terzan 1: The Neutron Star X-Ray Transient X1732–304 in Quiescence , 2001, astro-ph/0111337.

[32]  A CHANDRA OBSERVATION OF THE NEUTRON STAR X-RAY TRANSIENT AND ECLIPSING BINARY MXB 1659-29 IN QUIESCENCE , 2002, astro-ph/0207094.

[33]  Carl J. Grillmair,et al.  The Main-Sequence Luminosity Function of Palomar 5 from the Hubble Space Telescope , 1996, astro-ph/0110411.

[34]  Ivan R. King,et al.  The structure of star clusters. III. Some simple dvriamical models , 1966 .

[35]  S. Anderson,et al.  The Rapid Burster in Liller 1: The Chandra X-Ray Position and a Search for an Infrared Counterpart , 2001, astro-ph/0106140.

[36]  Giampaolo Piotto,et al.  Omega Centauri, A Unique Window into Astrophysics , 2002 .

[37]  R. Elsner,et al.  CHANDRA X-RAY OBSERVATORY OBSERVATIONS OF THE GLOBULAR CLUSTER M28 AND ITS MILLISECOND PULSAR PSR B1821-24 , 2002 .

[38]  Astronomy,et al.  Blue Straggler Stars: A Direct Comparison of Star Counts and Population Ratios in Six Galactic Globular Clusters , 2003, astro-ph/0301261.

[39]  B. Gendre,et al.  An XMM-Newton observation of the globular cluster Omega Centauri , 2002, astro-ph/0212173.

[40]  Robert E. Rutledge,et al.  Variable Thermal Emission from Aquila X-1 in Quiescence , 2002, astro-ph/0204196.

[41]  M. Colpi,et al.  Charting the Temperature of the Hot Neutron Star in a Soft X-Ray Transient , 2000, astro-ph/0010572.

[42]  R. Gilliland,et al.  An Extensive Census of Hubble Space Telescope Counterparts to Chandra X-Ray Sources in the Globular Cluster 47 Tucanae. I. Astrometry and Photometry , 2003, astro-ph/0307187.

[43]  S. Borgani,et al.  First Results from the X-Ray and Optical Survey of the Chandra Deep Field South , 2000, astro-ph/0007240.

[44]  A. Cool,et al.  Discovery of candidate cataclysmic variables in the post-core-collapse globular cluster NGC 6397 , 1995 .

[45]  L. Bildsten,et al.  The Quiescent X-Ray Spectrum of the Neutron Star in Centaurus X-4 Observed with Chandra/ACIS-S , 2000, astro-ph/0012400.

[46]  F. Camilo,et al.  Chandra Study of a Complete Sample of Millisecond Pulsars in 47 Tucanae and NGC 6397 , 2002, astro-ph/0208280.

[47]  D. Helfand,et al.  The Origin and Evolution of Neutron Stars , 1987 .

[48]  L. Pulone,et al.  VLT observations of the peculiar globular cluster NGC 6712, II: luminosity and mass functions , 2001, astro-ph/0103354.

[49]  E. Phinney,et al.  STATISTICS OF PULSARS IN GLOBULAR CLUSTERS , 1992 .

[50]  S. Rappaport,et al.  SUBMITTED TO APJ Preprint typeset using L ATEX style emulateapj v. 21/08/00 THE GALACTIC POPULATION OF LOW- AND INTERMEDIATE-MASS X-RAY BINARIES , 2003 .

[51]  J. Grindlay,et al.  Hubble Space Telescope/NICMOS Observations of Terzan 5: Stellar Content and Structure of the Core , 2002, astro-ph/0202059.

[52]  M. Bellazzini,et al.  Deep Hubble Space Telescope WFPC2 Photometry of NGC 288. I. Binary Systems and Blue Stragglers , 2001, astro-ph/0112343.

[53]  J. Grindlay,et al.  An X-ray survey of globular clusters and their X-ray luminosity function. , 1983 .

[54]  G. Clark.  X-ray binaries in globular clusters , 1975 .

[55]  S. Rappaport,et al.  Predictions of a population of cataclysmic variables in globular clusters , 1994 .

[56]  An Extensive Census of Hubble Space Telescope Counterparts to Chandra X-Ray Sources in the Globular Cluster 47 Tucanae. II. Time Series and Analysis , 2003, astro-ph/0307189.

[57]  Mit,et al.  Identification of the Optical and Quiescent Counterparts to the Bright X-Ray Transient in NGC 6440 , 2001, astro-ph/0111213.

[58]  Low-mass X-ray binaries in globular clusters: A new metallicity effect , 1994, astro-ph/9407086.

[59]  J. Grindlay,et al.  X-Ray Studies of Two Neutron Stars in 47 Tucanae: Toward Constraints on the Equation of State , 2003, astro-ph/0301235.

[60]  William E. Harris,et al.  A Catalog of Parameters for Globular Clusters in the Milky Way , 1996 .

[61]  M. Nowak,et al.  Hiding in Plain Sight: Chandra Observations of the Quiescent Neutron Star 4U 2129+47 in Eclipse , 2002, astro-ph/0204503.

[62]  Duane A. Liedahl,et al.  New Calculations of Fe L-Shell X-Ray Spectra in High-Temperature Plasmas , 1995 .

[63]  J. Grindlay,et al.  Spectroscopic Identification of Probable Cataclysmic Variables in the Globular Cluster NGC 6397 , 1995 .

[64]  THE BRIGHT OPTICAL COMPANION TO THE ECLIPSING MILLISECOND PULSAR IN NGC 6397 , 2001, astro-ph/0108252.

[65]  J. Grindlay,et al.  Identification of the Low-Mass X-Ray Binary and Faint X-Ray Sources in NGC 6652 , 2001, astro-ph/0107513.

[66]  F. Verbunt.  X-ray sources in globular clusters , 1984, astro-ph/0111439.

[67]  Berkeley,et al.  The Luminosity Function of the Globular Cluster NGC 6397 near the Limit of Hydrogen Burning , 1997, astro-ph/9711004.

[68]  Neutron Star Mass Measurements. I. Radio Pulsars , 1998, astro-ph/9803260.

[69]  E. S. Phinney,et al.  BINARIES IN GLOBULAR CLUSTERS , 1992, astro-ph/9710262.