Field Low-mass X-ray Binaries Research Articles

Properties of intra-cluster low-mass X-ray binaries in Fornax globular clusters

Aims. We present a study of the intra-cluster population of low-mass X-ray binaries (LMXB) residing in globular clusters (GC) in the central 1 deg2 of the Fornax galaxy cluster. Differently from previous studies, which were restricted to the innermost regions of individual galaxies, this work is aimed at comparing the properties of the intra-cluster population of GC-LMXBs with those of the host galaxy. Methods. The data used in this work are a combination of the VLT Survey Telescope (VST) and Chandra observations. We performed a cross-match between the optical and the X-ray catalogue in order to identify the LMXBs residing in GCs. We divided the GC-LMXBs into host-galaxy and intra-cluster objects based on their distance from the nearest galaxy in terms of effective radius (Reff). We found 82 intra-cluster GC-LMXBs and 86 objects that are hosted in galaxies. As the formation of LMXBs also depends on the host GC colour, we performed a Gaussian mixture model to divide the population into red and blue GCs. Results. As has been found for the innermost regions of galaxies, LMXBs tend to form in red and bright GCs in intra-cluster space as well. We find, however, that the likelihood of a red GC to host an LMXB decreases with galactocentric distance, but it remains approximately constant for the blue GC population. Investigating the X-ray properties of the LMXBs residing in GCs, we find a difference in the X-ray luminosity function between the intra-cluster and host-galaxy sample: both follow a power-law down to ∼8.5 × 1037 erg s−1, which is consistent with field LMXBs for the intra-cluster sample, while the latter agree with previous estimates for LMXBs in GCs. We observe a deficiency of bright LMXBs in blue intra-cluster GCs, however. This might indicate a lack of black hole binaries in metal-poor systems. We further investigated the spectral properties of the GC-LMXBs through their hardness-ratio. We detect a tentative difference in the hardness ratio of two populations, where the intra-cluster GC-LMXBs appear to have harder spectra than the host-galaxy objects. We find the same trend when we compare red and blue GC-LMXBs: the spectra of the blue sample are harder spectra than those of the red sample. This result could suggest a relation between the spectral properties of LMXBs and the host GC colour and therefore its metallicity. We discuss the possibilities of spatial biases due to uncertainties in the X-ray spectral response correction and due to contamination by background active galactic nuclei.

X-Ray Binary Luminosity Function Scaling Relations in Elliptical Galaxies: Evidence for Globular Cluster Seeding of Low-mass X-Ray Binaries in Galactic Fields

We investigate X-ray binary (XRB) luminosity function (XLF) scaling relations for Chandra detected populations of low-mass XRBs (LMXBs) within the footprints of 24 early-type galaxies. Our sample includes Chandra and HST observed galaxies at D < 25 Mpc that have estimates of the globular cluster (GC) specific frequency (SN) reported in the literature. As such, we are able to directly classify X-ray-detected sources as being either coincident with unrelated background/foreground objects, GCs, or sources that are within the fields of the galaxy targets. We model the GC and field LMXB population XLFs for all galaxies separately, and then construct global models characterizing how the LMXB XLFs vary with galaxy stellar mass and SN. We find that our field LMXB XLF models require a component that scales with SN, and has a shape consistent with that found for the GC LMXB XLF. We take this to indicate that GCs are "seeding" the galactic field LMXB population, through the ejection of GC-LMXBs and/or the diffusion of the GCs in the galactic fields themselves. However, we also find that an important LMXB XLF component is required for all galaxies that scales with stellar mass, implying that a substantial population of LMXBs are formed "in situ," which dominates the LMXB population emission for galaxies with SN < 2. For the first time, we provide a framework quantifying how directly-associated GC LMXBs, GC-seeded LMXBs, and in-situ LMXBs contribute to LMXB XLFs in the broader early-type galaxy population.

An Excess of Low-mass X-Ray Binaries in the Outer Halo of NGC 4472

Abstract We present new Chandra observations of the outer halo of the giant elliptical galaxy NGC 4472 (M49) in the Virgo Cluster. The data extend to 130 kpc (28′), and have a combined exposure time of 150 ks. After identifying optical counterparts using the Next Generation Virgo Cluster Survey to remove background active galactic nuclei and globular cluster (GC) sources, and correcting for completeness, we find that the number of field low-mass X-ray binaries (LMXBs) per unit stellar V-band light increases significantly with the galactocentric radius. Because the flux limit of the complete sample corresponds to the Eddington limit for neutron stars in NGC 4472, many of the ∼90 field LMXBs in this sample could host black holes. The excess of field LMXBs at large galactocentric radii may be partially caused by natal kicks on black holes and neutron stars in binary systems in the inner part of the galaxy. Furthermore, because the metallicity in the halo of NGC 4472 strongly decreases toward larger galactocentric radii, the number of field LMXBs per unit stellar mass is anticorrelated with metallicity, opposite to what is observed in GCs. Another way to explain the spatial distribution of field LMXBs is therefore a reversed metallicity effect, although we have not identified a mechanism to explain this in terms of stellar and binary evolution.

Deep Chandra observations of NGC 7457, the X-ray point source populations of a low mass early-type galaxy

We present the X-ray point source population of NGC 7457 based on 124 ks of Chandra observations. Previous deep Chandra observations of low mass X-ray binaries (LMXBs) in early-type galaxies have typically targeted the large populations of massive galaxies. NGC 7457 is a nearby, early-type galaxy with a stellar luminosity of $1.7\times10^{10} L_{K\odot}$, allowing us to investigate the populations in a relatively low mass galaxy. We classify the detected X-ray sources into field LMXBs, globular cluster LMXBs, and background AGN based on identifying optical counterparts in new HST/ACS images. We detect 10 field LMXBs within the $r_{ext}$ ellipse of NGC 7457 (with semi-major axis $\sim$ 9.1 kpc, ellipticity = 0.55). The corresponding number of LMXBs with $L_{x}>2\times10^{37}erg/s$ per stellar luminosity is consistent with that observed in more massive galaxies, $\sim 7$ per $10^{10} L_{K\odot}$. We detect a small globular cluster population in these HST data and show that its colour distribution is likely bimodal and that its specific frequency is similar to that of other early type galaxies. However, no X-ray emission is detected from any of these clusters. Using published data for other galaxies, we show that this non-detection is consistent with the small stellar mass of these clusters. We estimate that 0.11 (and 0.03) LMXBs are expected per $10^{6}M_{\odot}$ in metal-rich (and metal-poor) globular clusters. This corresponds to 1100 (and 330) LMXBs per $10^{10} L_{K\odot}$, highlighting the enhanced formation efficiency of LMXBs in globular clusters. A nuclear X-ray source is detected with $L_{x}$ varying from $2.8-6.8\times10^{38}erg/s$. Combining this $L_{x}$ with a published dynamical mass estimate for the central SMBH in NGC 7457, we find that $L_{x}/L_{Edd}$ varies from $0.5-1.3\times10^{-6}$.

THE X-RAY LUMINOSITY FUNCTION OF LOW MASS X-RAY BINARIES IN EARLY-TYPE GALAXIES, THEIR METAL-RICH, AND METAL-POOR GLOBULAR CLUSTERS

ABSTRACT We present the X-ray luminosity function (XLF) of low-mass X-ray binaries (LMXBs) in the globular clusters (GCs) and fields of seven early-type galaxies. These galaxies are selected to have both deep Chandra observations, which allow their LMXB populations to be observed to X-ray luminosities of 1037–1038 erg s−1, and Hubble Space Telescope optical mosaics that enable the X-ray sources to be separated into field LMXBs, GC LMXBs, and contaminating background and foreground sources. We find that at all luminosities the number of field LMXBs per stellar mass is similar in these galaxies. This suggests that the field LMXB populations in these galaxies are not effected by the GC specific frequency, and that properties such as binary fraction and the stellar initial mass function are either similar across the sample or change in a way that does not affect the number of LMXBs. We compare the XLF of the field LMXBs to that of the GC LMXBs and find that they are significantly different with a p-value of 3 × 10−6 (equivalent to 4.7σ for a normal distribution). The difference is such that the XLF of the GC LMXBs is flatter than that of the field LMXBs, with the GCs hosting relatively more bright sources and fewer faint sources. A comparison of the XLF of the metal-rich and metal-poor GCs hints that the metal-poor clusters may have more bright LMXBs, but the difference is not statistically significant.

THE MEGASECONDCHANDRAX-RAY VISIONARY PROJECT OBSERVATION OF NGC 3115. III. LUMINOSITY FUNCTIONS OF LMXBS AND DEPENDENCE ON STELLAR ENVIRONMENTS

We have studied the X-ray luminosity function (XLF) of low-mass X-ray binaries (LMXBs) in the nearby lenticular galaxy NGC 3115, using the Megasecond Chandra X-Ray Visionary Project Observation. With a total exposure time of ~1.1 Ms, we constructed the XLF down to a limiting luminosity of ~10^36 erg/s, much deeper than typically reached for other early-type galaxies. We found significant flattening of the overall LMXB XLF from dN/dL \propto L^{-2.2\pm0.4} above 5.5x10^37 erg/s to dN/dL \propto L^{-1.0\pm0.1} below it, though we could not rule out a fit with a higher break at ~1.6x10^38 erg/s. We also found evidence that the XLF of LMXBs in globular clusters (GCs) is overall flatter than that of field LMXBs. Thus our results for this galaxy do not support the idea that all LMXBs are formed in GCs. The XLF of field LMXBs seems to show spatial variation, with the XLF in the inner region of the galaxy being flatter than that in the outer region, probably due to contamination of LMXBs from undetected and/or disrupted GCs in the inner region. The XLF in the outer region is probably the XLF of primordial field LMXBs, exhibiting dN/dL \propto L^{-1.2\pm0.1} up to a break close to the Eddington limit of neutron star LMXBs (~1.7x10^38 erg/s). The break of the GC LMXB XLF is lower, at ~1.1x10^37 erg/s. We also confirm previous findings that the metal-rich/red GCs are more likely to host LMXBs than the metal-poor/blue GCs, which is more significant for more luminous LMXBs, and that more massive GCs are more likely to host LMXBs.

THE X-RAY LUMINOSITY FUNCTIONS OF FIELD LOW-MASS X-RAY BINARIES IN EARLY-TYPE GALAXIES: EVIDENCE FOR A STELLAR AGE DEPENDENCE

We present direct constraints on how the formation of low-mass X-ray binary (LMXB) populations in galactic fields depends on stellar age. In this pilot study, we utilize Chandra and Hubble Space Telescope (HST) data to detect and characterize the X-ray point source populations of three nearby early-type galaxies: NGC 3115, 3379, and 3384. The luminosity-weighted stellar ages of our sample span 3-10 Gyr. X-ray binary population synthesis models predict that the field LMXBs associated with younger stellar populations should be more numerous and luminous per unit stellar mass than older populations due to the evolution of LMXB donor star masses. Crucially, the combination of deep Chandra and HST observations allows us to test directly this prediction by identifying and removing counterparts to X-ray point sources that are unrelated to the field LMXB populations, including LMXBs that are formed dynamically in globular clusters, Galactic stars, and background AGN/galaxies. We find that the "young" early-type galaxy NGC 3384 (~2-5 Gyr) has an excess of luminous field LMXBs (L_X > (5-10) x 10^37 erg/s) per unit K-band luminosity (L_K; a proxy for stellar mass) than the "old" early-type galaxies NGC 3115 and 3379 (~8-10 Gyr), which results in a factor of ~2-3 excess of LX/LK for NGC 3384. This result is consistent with the X-ray binary population synthesis model predictions; however, our small galaxy sample size does not allow us to draw definitive conclusions on the evolution field LMXBs in general. We discuss how future surveys of larger galaxy samples that combine deep Chandra and HST data could provide a powerful new benchmark for calibrating X-ray binary population synthesis models.

THE TWO-DIMENSIONAL SPATIAL DISTRIBUTIONS OF THE GLOBULAR CLUSTERS AND LOW-MASS X-RAY BINARIES OF NGC 4649

We report significant anisotropies in the projected two-dimensional (2D) spatial distributions of Globular Clusters (GCs) of the giant Virgo elliptical galaxy NGC4649 (M60). Similar features are found in the 2D distribution of low-mass X-ray binaries (LMXBs), both associated with GCs and in the stellar field. Deviations from azimuthal symmetry suggest an arc-like excess of GCs extending north at 4-15 kpc galactocentric radii in the eastern side of major axis of NGC4649. This feature is more prominent for red GCs, but still persists in the 2D distribution of blue GCs. High and low luminosity GCs also show some segregation along this arc, with high-luminosity GCs preferentially located in the southern end and low-luminosity GCs in the northern section of the arc. GC-LMXBs follow the anisotropy of red-GCs, where most of them reside; however, a significant overdensity of (high-luminosity) field LMXBs is present to the south of the GC arc. These results suggest that NGC4649 has experienced mergers and/or multiple accretions of less massive satellite galaxies during its evolution, of which the GCs in the arc may be the fossil remnant. We speculate that the observed anisotropy in the field LMXB spatial distribution indicates that these X-ray binaries may be the remnants of a star formation event connected with the merger, or maybe be ejected from the parent red GCs, if the bulk motion of these clusters is significantly affected by dynamical friction. We also detect a luminosity enhancement in the X-ray source population of the companion spiral galaxy NGC4647. We suggest that these may be younger high mass X-ray binaries formed as a result of the tidal interaction of this galaxy with NGC4649.

THE RADIAL DISTRIBUTION OF X-RAY BINARIES AND GLOBULAR CLUSTERS IN NGC 4649 AND THEIR RELATION WITH THE LOCAL STELLAR MASS DENSITY

We investigate the radial distribution of the low-mass X-ray binary (LMXB) population in the elliptical galaxy NGC 4649, using Chandra and Hubble data to separate the field and globular cluster (GC) populations. GCs with LMXBs have the same radial distribution as the parent red and blue GCs. The radial profile of field LMXBs follows the V-band profile within the D25 of NGC 4649. Using the spatial information provided by our data, we find that the global galaxy-wide relations between cumulative number and luminosity of LMXBs and the integrated stellar mass hold on local scales within D25. An excess of field LMXBs with respect to the V-light is observed in the galaxy's outskirt, which may be partially due to unidentified GC sources or to a rejuvenated field LMXB population caused by past merging interactions.

Low-mass X-ray binary populations in galaxy outskirts: Globular clusters and supernova kicks

For the first time, we have systematically explored the population of discrete X-ray sources in the outskirts of early-type galaxies. Based on a broad sample of 20 galaxies observed with Chandra we detected overdensity of X-ray sources in their outskirts. The overdensity appears as halos of resolved sources around the galaxies. These halos are broader than the stellar light, extending out to at least ~ 10 Re (Re is the effective radius). These halos are composed of sources fainter than ~5E38 erg/s, whereas the more luminous sources appear to follow the distribution of the stellar light, suggesting that the excess source population consists of neutron star binaries. Dividing the galaxy sample into four groups according to their stellar mass and specific frequency of globular clusters, we find that the extended halos are present in all groups except for the low-mass galaxies with low globular cluster content. We propose that the extended halos may be comprised of two independent components, low-mass X-ray binaries (LMXBs) located in globular clusters (GCs), which are known to have a wider distribution than the stellar light, and neutron star (NS) LMXBs kicked out of the main body of the parent galaxy by supernova explosions. The available deep optical and X-ray data of NGC 4365 support this conclusion. For this galaxy we identified 60.1+/-10.8 excess sources in the 4-10 Re region of which ~ 40% are located in GCs, whereas ~ 60% are field LMXBs. We interpret the latter as kicked NS LMXBs. We discuss the implications of these results for the natal kick distributions of black holes and neutron stars.

Luminosity functions of LMXBs in different stellar environments

Based on the archival data from the Chandra observations of nearby galaxies, we study different sub populations of low-mass X-ray binaries (LMXBs) - dynamically formed systems in globular clusters (GCs) and in the nucleus of M31 and (presumably primordial) X-ray binaries in the fields of galaxies. Our aim is to produce accurate luminosity distributions of X-ray binaries in different environments, suitable for quantitative comparison with each other and with the output of population synthesis calculations. Our sample includes seven nearby galaxies (M31, Maffei 1, Centaurus A,M81, NGC 3379, NGC 4697, and NGC 4278) and the Milky Way, which together provide relatively uniform coverage down to the luminosity limit of E35 erg/s. In total we have detected 185 LMXBs associated with GCs, 35 X-ray sources in the nucleus of M31, and 998 field sources of which ~ 365 are expected to be background AGN. We combine these data, taking special care to accurately account for X-ray and optical incompleteness corrections and the removal of the contamination from the cosmic X-ray background sources, to produce luminosity distributions of X-ray binaries in different environments to far greater accuracy than has been obtained previously. We found that luminosity distributions of GC and field LMXBs differ throughout the entire luminosity range, the fraction of faint (log(Lx) < 37) sources among the former being ~ 4 times less than in the field population. The X-ray luminosity function (XLF) of sources in the nucleus of M31 is similar to that of GC sources at the faint end but differs at the bright end, with the M31 nucleus hosting significantly fewer bright sources. We discuss the possible origin and potential implications of these results.

FIELD AND GLOBULAR CLUSTER LOW-MASS X-RAY BINARIES IN NGC 4278

We report a detailed spectral analysis of the population of low-mass X-ray binaries (LMXBs) detected in the elliptical galaxy NGC~4278 with Chandra. Seven luminous sources were studied individually, four in globular clusters (GCs), and three in the stellar field. The range of (0.3-8 keV) $L_X$ for these sources suggests that they may be black hole binaries (BHBs). Comparison of our results with simulations allows us to discriminate between disk and power-law dominated emission, pointing to spectral/luminosity variability, reminiscent of Galactic BHBs. The BH masses derived from a comparison of our spectral results with the $L_X \sim T^4_{in}$ relation of Galactic BHBs are in the 5-15 $M_{\odot}$ range, as observed in the Milky Way. The analysis of joint spectra of sources selected in three luminosity ranges suggests that while the high luminosity sources have prominent thermal disk emission components, power-law components are likely to be important in the mid and low-luminosity spectra. Comparing low-luminosity average spectra, we find a relatively larger $N_H$ in the GC spectrum; we speculate that this may point to either a metallicity effect, or to intrinsic physical differences between field and GC accreting binaries. Analysis of average sample properties uncover a previously unreported $L_X - R_G$ correlation (where $R_G$ is the galactocentric radius) in the GC-LMXB sample, implying richer LMXB populations in more central GCs. No such trend is seen in the field LMXB sample. We can exclude that the GC $L_X - R_G$ correlation is the by-product of a luminosity effect, and suggest that it may be related to the presence of more compact GCs at smaller galactocentric radii, fostering more efficient binary formation.

X-RAY EMISSION FROM THE SOMBRERO GALAXY: DISCRETE SOURCES

We present a study of discrete X-ray sources in and around the bulge-dominated, massive Sa galaxy, Sombrero (M104), based on new and archival Chandra observations with a total exposure of ~200 ks. With a detection limit of L_X = 1E37 erg/s and a field of view covering a galactocentric radius of ~30 kpc (11.5 arcminute), 383 sources are detected. Cross-correlation with Spitler et al.'s catalogue of Sombrero globular clusters (GCs) identified from HST/ACS observations reveals 41 X-rays sources in GCs, presumably low-mass X-ray binaries (LMXBs). We quantify the differential luminosity functions (LFs) for both the detected GC and field LMXBs, whose power-low indices (~1.1 for the GC-LF and ~1.6 for field-LF) are consistent with previous studies for elliptical galaxies. With precise sky positions of the GCs without a detected X-ray source, we further quantify, through a fluctuation analysis, the GC LF at fainter luminosities down to 1E35 erg/s. The derived index rules out a faint-end slope flatter than 1.1 at a 2 sigma significance, contrary to recent findings in several elliptical galaxies and the bulge of M31. On the other hand, the 2-6 keV unresolved emission places a tight constraint on the field LF, implying a flattened index of ~1.0 below 1E37 erg/s. We also detect 101 sources in the halo of Sombrero. The presence of these sources cannot be interpreted as galactic LMXBs whose spatial distribution empirically follows the starlight. Their number is also higher than the expected number of cosmic AGNs (52+/-11 [1 sigma]) whose surface density is constrained by deep X-ray surveys. We suggest that either the cosmic X-ray background is unusually high in the direction of Sombrero, or a distinct population of X-ray sources is present in the halo of Sombrero.

COMPARING GC AND FIELD LMXBs IN ELLIPTICAL GALAXIES WITH DEEPCHANDRAANDHUBBLEDATA

We present a statistical study of the low-mass X-ray binary (LMXB) populations of three nearby, old elliptical galaxies: NGC 3379, NGC 4278, and NGC 4697. With a cumulative ~1 Ms Chandra ACIS observing time, we detect 90-170 LMXBs within the D25 ellipse of each galaxy. Cross-correlating Chandra X-ray sources and HST optical sources, we identify 75 globular cluster (GC) LMXBs and 112 field LMXBs with LX > 1036 erg s–1 (detections of these populations are 90% complete down to luminosities in the range of 6 × 1036 to 1.5 × 1037 erg s–1). At the higher luminosities explored in previous studies, the statistics of this sample are consistent with the properties of GC-LMXBs reported in the literature. In the low-luminosity range allowed by our deeper data (LX < 5 × 1037 erg s–1), we find a significant relative lack of GC-LMXBs, when compared with field sources. Using the co-added sample from the three galaxies, we find that the incompleteness-corrected X-ray luminosity functions (XLFs) of GC and field LMXBs differ at ~4σ significance at LX < 5 × 1037 erg s–1. As previously reported, these XLFs are consistent at higher luminosities. The presently available theoretical models for LMXB formation and evolution in clusters are not sophisticated enough to provide a definite explanation for the shape of the observed GC-LMXB XLF. Our observations may indicate a potential predominance of GC-LMXBs with donors evolved beyond the main sequence, when compared to current models, but their efficient formation requires relatively high initial binary fractions in clusters. The field LMXB XLF can be fitted with either a single power-law model plus a localized excess at a luminosity of (5-6) × 1037 erg s–1, or a broken power law with a similar low-luminosity break. This XLF may be explained with NS-red-giant LMXBs, contributing to ~15% of total LMXBs population at ~5 × 1037 erg s–1. The difference in the GC and field XLFs is consistent with different origins and/or evolutionary paths between the two LMXB populations, although a fraction of the field sources are likely to have originated in GCs.

TRANSIENT LOW-MASS X-RAY BINARY POPULATIONS IN ELLIPTICAL GALAXIES NGC 3379 AND NGC 4278

We propose a physically motivated and self-consistent prescription for the modeling of transient neutron star low-mass X-ray binary (LMXB) properties, such as duty cycle (DC), outburst duration, and recurrence time. We apply this prescription to the population synthesis models of field LMXBs presented by Fragos et al., and compare the transient LMXB population to the Chandra X-ray survey of the two elliptical galaxies NGC 3379 and NGC 4278, which revealed several transient sources. We are able to exclude models with a constant DC for all transient systems, while models with a variable DC based on the properties of each system are consistent with the observed transient populations. We predict that the majority of the observed transient sources in these two galaxies are LMXBs with red giant donors. Finally, our comparison suggests that transient LMXBs are very rare in globular clusters (GCs), and thus the number of identified transient LMXBs may be used as a tracer of the relative contribution of field and GC LMXB populations.

Models for Low‐Mass X‐Ray Binaries in the Elliptical Galaxies NGC 3379 and NGC 4278: Comparison with Observations

We present theoretical models for the formation and evolution of populations of low-mass X-ray binaries (LMXB) in the two elliptical galaxies NGC 3379 and NGC 4278. The models are calculated with the recently updated StarTrack code (Belczynski et al. 2007), assuming only a primordial galactic field LMXB population. StarTrack is an advanced population synthesis code that has been tested

Probing the Formation of Low‐Mass X‐Ray Binaries in Globular Clusters and the Field

We present an analysis of low mass X-ray binaries (LMXBs) and globular clusters (GCs) in five galaxies using Chandra, and HST data. Of the 186 LMXBs within the optical fields 71 are in GCs, confirming that LMXBs are formed particularly efficiently in GCs. However, there is no statistically significant correlation between the distance of a GC from the center of its host galaxy and its LMXB hosting probability. Metal-rich GCs are 3.4 times more likely to host LMXBs than metal-poor ones. Intriguingly, the LMXBs in NGC 1399 are preferentially in the reddest GCs of the metal-rich subsystem. This indicates that the red peak of the color distribution itself encompasses GCs with a range of enrichment histories. The strength of this effect varies from galaxy to galaxy, possibly indicating differences in their metal-enrichment histories. Field LMXBs in our program galaxies are more concentrated towards the center of their host galaxies than GC-LMXBs. This suggests that a majority of field LMXBs are formed in situ and have not escaped from current GCs. This is consistent with previous specific frequency based studies. The brightest LMXBs in GCs appear to be preferentially associated with luminous, metal-rich clusters. We show that it is probable that some of these GCs host multiple bright LMXBs, while the probability is much lower for metal-poor GCs. This implies that LMXBs in more metal-rich cluster systems should reveal a longer high luminosity X-ray tail, and show less X-ray variability than metal-poor cluster populations. (abridged)

Low‐Mass X‐Ray Binaries in Six Elliptical Galaxies: Connection to Globular Clusters

We present a systematic study of the low-mass X-ray binary (LMXB) populations of six elliptical galaxies, aimed at investigating the detected LMXB-globular cluster (GC) connection. We utilize Chandra archival data to identify X-ray point sources and HST archival data supplemented by ground observations to identify 6173 GCs. After screening and cross-matching, we associate 209 LMXBs with red GC (RGCs) and 76 LMXBs with blue GCs (BGCs), while we find no optical GC counterpart for 258 LMXBs. This is the largest GC-LMXB sample studied so far. We confirm previous reports suggesting that the fraction of GCs associated with LMXBs is ~3 times larger in RGCs than in BGCs, indicating that metallicity is a primary factor in the GC LMXB formation. We find that GCs located near the galaxy center have a higher probability of harboring LMXBs than those in the outskirts, suggesting that there must be another parameter (in addition to metallicity) governing LMXB formation in GCs. This second parameter, dependent on the galactocentric distance, may be a distance dependent encounter rate. We find no significant differences in the shape of X-ray luminosity function, LX/LV distribution, X-ray spectra among RGC, BGC, and field LMXBs. The similarity of the X-ray spectra is inconsistent with the irradiation-induced stellar wind model prediction. The similarity of the X-ray luminosity functions (XLFs) of GC LMXBs and field LMXBs indicates that there is no significant difference in the fraction of black hole binaries present. We cannot either prove or reject the hypothesis that all LMXBs were formed in GCs.

Low Mass X-ray Binaries and Globular Clusters in Early-Type Galaxies

Chandra observations have allowed the detection of a large number of low mass X-ray binaries (LMXBs) in early-type galaxies. Comparison to catalogs of globular clusters (GCs) from Hubble Space Telescope observations have shown that a high fraction of the LMXBs in early-type galaxies are associated with GCs. The fraction of LMXBs associated with globular clusters increases along the Hubble sequence from spiral bulges to S0s to Es to cDs. On the other hand, the fraction of globular clusters which contain X-ray sources appears to be roughly constant ( $\sim$ 4% for $L_X \gtrsim 10^{38}$ ergs/s, $\sim$ 10% for $L_X \gtrsim 10^{37}$ ergs/s). There is a strong tendency for the X-ray sources to be associated with the optically more luminous GCs. There is a trend for the X-ray sources to be found preferentially in redder, more metal-rich GCs, which is independent of optical luminosity correlation. The relative role of formation of LMXBs in GCs and in situ formation in the field is uncertain. One of the best ways to study this is to compare the spatial distribution of GC-LMXBs, field LMXBs, GCs, and optical light in the galaxies. Theoretical models and results of fits to the observed distributions are presented.

On the Nature of X-Ray Sources in Early-Type Galaxies

We show that the observed relationship between the fraction of low-mass X-ray binaries (LMXBs) found in globular clusters (GCs) and the GC-specific frequency for early-type galaxies is consistent with an LMXB formation model in which the field population of LMXBs is formed in situ via primordial binary formation. The suggestion that a significant fraction of the field LMXB population in early-type galaxies was formed in GCs is not required by the data. Finally, we discuss observational studies that will test this model more thoroughly.