arXiv subjects

Philip J. Humphrey

Publications and source records attributed to Philip J. Humphrey.

At least 19 recordsLinked to original sources

arXiv2013

Sterile neutrino dark matter bounds from galaxies of the Local Group

We show that the canonical oscillation-based (non-resonant) production of sterile neutrino dark matter is inconsistent at $>99$% confidence with observations of galaxies in the Local Group. We set lower limits on the non-resonant sterile neutrino mass of $2.5$ keV (equivalent to $0.7$ keV thermal mass) using phase-space densities derived for dwarf satellite galaxies of the Milky Way, as well as limits of $8.8$ keV (equivalent to $1.8$ keV thermal mass) based on subhalo counts of $N$-body simulations of M 31 analogues. Combined with improved upper mass limits derived from significantly deeper X-ray data of M 31 with full consideration for background variations, we show that there remains little room for non-resonant production if sterile neutrinos are to explain $100$% of the dark matter abundance. Resonant and non-oscillation sterile neutrino production remain viable mechanisms for generating sufficient dark matter sterile neutrinos.

astro-ph.CO↗

arXiv2013

Narrow Band X-ray Photometry as a Tool for Studying Galaxy and Cluster Mass Distributions

We explore the utility of narrow band X-ray surface photometry as a tool for making fully Bayesian, hydrostatic mass measurements of clusters of galaxies, groups and early-type galaxies. We demonstrate that it is sufficient to measure the surface photometry with the Chandra X-ray observatory in only three (rest frame) bands (0.5--0.9 keV, 0.9--2.0 keV and 2.0--7.0 keV) in order to constrain the temperature, density and abundance of the hot interstellar medium (ISM). Adopting parametrized models for the mass distribution and radial entropy profile and assuming spherical symmetry, we show that the constraints on the mass and thermodynamic properties of the ISM that are obtained by fitting data from all three bands simultaneously are comparable to those obtained by fitting similar models to the temperature and density profiles derived from spatially resolved spectroscopy, as is typically done. We demonstrate that the constraints can be significantly tightened when exploiting a recently derived, empirical relationship between the gas fraction and the entropy profile at large scales, eliminating arbitrary extrapolations at large radii. This "Scaled Adiabatic Model" (ScAM) is well suited to modest signal-to-noise data, and we show that accurate, precise measurements of the global system properties are inferred when employing it to fit data from even very shallow, snapshot X-ray observations. The well-defined asymptotic behaviour of the model also makes it ideally suited for use in Sunyaev-Zeldovich studies of galaxy clusters.

astro-ph.CO↗

arXiv2012

Reconciling stellar dynamical and hydrostatic X-ray mass measurements of an elliptical galaxy with gas rotation, turbulence and magnetic fields

Recent hydrostatic X-ray studies of the hot interstellar medium (ISM) in early-type galaxies underestimate the gravitating mass as compared to stellar dynamics, implying modest, but significant deviations from exact hydrostatic equilibrium. We present a method for combining X-ray measurements and stellar dynamical constraints in the context of Bayesian statistics that allows the radial distribution of the implied nonthermal pressure or bulk motions in the hot ISM to be constrained. We demonstrate the accuracy of the method with hydrodynamical simulations tailored to produce a realistic galaxy model. Applying the method to the nearby elliptical galaxy NGC4649, we find a significant but subdominant nonthermal pressure fraction (0.27+/-0.06) in the central (<5 kpc) part of the galaxy, similar to the level of deviations from hydrostatic equilibrium expected in galaxy clusters. Plausible sources of systematic error, if important, may reduce this fraction. This would imply >360 km/s random turbulence or a magnetic field B=(39+/-6)(n_e/0.1 cm^{-3})^{0.59+/-0.09} muG, whereas gas rotation alone is unlikely to explain the detailed nonthermal profile. Future observations with Astro-H will allow turbulence or gas rotation at this level to be detected.

astro-ph.CO↗

arXiv2012

The ELIXR Galaxy Survey. II: Baryons and Dark Matter in an Isolated Elliptical Galaxy

The Elliptical Isolated X-ray (ElIXr) Galaxy Survey is a volume-limited (<110Mpc) study of optically selected, isolated, Lstar elliptical galaxies, to provide an X-ray census of galaxy-scale (virial mass, Mvir < 1e13 Msun) objects, and identify candidates for detailed hydrostatic mass modelling. In this paper, we present a Chandra and XMM study of one such candidate, NGC1521, and constrain its distribution of dark and baryonic matter. We find a morphologically relaxed hot gas halo, extending almost to R500, that is well described by hydrostatic models similar to the benchmark, baryonically closed, Milky Way-mass elliptical galaxy NGC720. We obtain good constraints on the enclosed gravitating mass (M500=3.8e12+/-1e12 Msun, slightly higher than NGC\thin 720), and baryon fraction (fb500=0.13+/-0.03). We confirm at 8.2-sigma the presence of a dark matter (DM) halo consistent with LCDM. Assuming a Navarro-Frenk-White DM profile, our self-consistent, physical model enables meaningful constraints beyond R500, revealing that most of the baryons are in the hot gas. Within the virial radius, fb is consistent with the Cosmic mean, suggesting that the predicted massive, quasi-hydrostatic gas halos may be more common than previously thought. We confirm that the DM and stars conspire to produce an approximately powerlaw total mass profile (rho \propto r^-alpha) that follows the recently discovered scaling relation between alpha and optical effective radius. Our conclusions are insensitive to modest, observationally motivated, deviations from hydrostatic equilibrium. Finally, after correcting for the enclosed gas fraction, the entropy profile is close to the self-similar prediction of gravitational structure formation simulations, as observed in massive galaxy clusters.

astro-ph.CO↗

arXiv2012

Tracing the Gas to the Virial Radius (R100) in a Fossil Group

We present a Chandra, Suzaku and Rosat study of the hot Intra Group Medium (IGrM) of the relaxed fossil group/ poor cluster RXJ1159+5531. This group exhibits an advantageous combination of flat surface brightness profile, high luminosity and optimal distance, allowing the gas to be detected out to the virial radius (Rvir=R108=1100 kpc) in a single Suzaku pointing, while the complementary Chandra data reveal a round morphology and relaxed IGrM image down to kpc scales. We measure the IGrM entropy profile over 3 orders of magnitude in radius, including 3 data bins beyond 0.5R200 with azimuthal coverage (>30%). We find no evidence that the profile flattens at large scales (>R500), and when corrected for the enclosed gas fraction, the entropy profile is very close to the predictions from self-similar structure formation simulations, as seen in massive clusters. Within Rvir, we measure a baryon fraction of 0.17+/-0.02, consistent with the Cosmological value. These results are in sharp contrast to the gas behaviour at large scales recently reported in the Virgo and Perseus clusters, and indicate that substantial gas clumping cannot be ubiquitous near Rvir, at least in highly evolved (fossil) groups.

astro-ph.CO↗

arXiv2011

Spherically Averaging Ellipsoidal Galaxy Clusters in X-Ray and Sunyaev-Zel'dovich Studies: II. Biases

This is the second of two papers investigating the spherical averaging of ellipsoidal galaxy clusters in the context of X-ray and Sunyaev-Zel'dovich (SZ) observations. In the present study we quantify the orientation-average bias and scatter in observables that result from spherically averaging clusters described by ellipsoidal generalizations of the NFW profile or a nearly scale-free logarithmic potential. Although the mean biases are small and mostly <1%, the flattest cluster models generally have a significant mean bias; i.e., averaging over all orientations does not always eliminate projection biases. Substantial biases can result from different viewing orientations, where the integrated Compton-y parameter (Y_SZ) and the concentration have the largest scatter (as large as sigma ~10% for Y_SZ), and the emission-weighted temperature (T_X) has the smallest (sigma < ~0.5%). The very small scatter for T_X leads to Y_X and M_gas having virtually the same orientation biases. Substantial scatter is expected for individual clusters (up to sigma ~8%) in the correlation between Y_SZ and Y_X in comparison to the small mean bias (sigma < ~1%) applicable to a random sample of clusters of sufficient size. For ellipsoidal NFW models we show that the orientation bias for the total cluster mass attains a minimum near the radius r_2500 so that the spherically averaged mass computed at this radius is always within ~0.5% of the true value for any orientation. Finally, to facilitate the accounting for orientation bias in X-ray and SZ cluster studies, we provide cubic polynomial approximations to the mean orientation bias and 1-sigma scatter for each cluster observable as a function of axial ratio for the ellipsoidal NFW models.

astro-ph.CO↗

arXiv2011

Spherically Averaging Ellipsoidal Galaxy Clusters in X-Ray and Sunyaev-Zel'dovich Studies: I. Analytical Relations

This is the first of two papers investigating the deprojection and spherical averaging of ellipsoidal galaxy clusters. We specifically consider applications to hydrostatic X-ray and Sunyaev-Zel'dovich (SZ) studies, though many of the results also apply to isotropic dispersion-supported stellar dynamical systems. Here we present analytical formulas for galaxy clusters described by a gravitational potential that is a triaxial ellipsoid of constant shape and orientation. For this model type we show that the mass bias due to spherically averaging X-ray observations is independent of the temperature profile, and for the special case of a scale-free logarithmic potential, there is exactly zero mass bias for any shape, orientation, and temperature profile. The ratio of spherically averaged intracluster medium (ICM) pressures obtained from SZ and X-ray measurements depends only on the ICM intrinsic shape, projection orientation, and H_0, which provides another illustration of how cluster geometry can be recovered through a combination of X-ray and SZ measurements. We also demonstrate that Y_SZ and Y_X have different biases owing to spherical averaging, which leads to an offset in the spherically averaged Y_SZ - Y_X relation. A potentially useful application of the analytical formulas presented is to assess the error range of an observable (e.g., mass, Y_SZ) accounting for deviations from assumed spherical symmetry, without having to perform the ellipsoidal deprojection explicitly. Finally, for dedicated ellipsoidal studies, we also generalize the spherical onion peeling method to the triaxial case for a given shape and orientation.

astro-ph.CO↗

arXiv2011

Dark Matter in Elliptical Galaxies

We review X-ray constraints on dark matter in giant elliptical galaxies (10^{12} M_sun <~ M_vir <~ 10^{13} M_sun) obtained using the current generation of X-ray satellites, beginning with an overview of the physics of the hot interstellar medium and mass modeling methodology. Dark matter is now firmly established in many galaxies, with inferred NFW concentration parameters somewhat larger than the mean theoretical relation. X-ray observations confirm that the total mass profile (baryons+DM) is close to isothermal (M ~ r), and new evidence suggests a more general power-law relation for the slope of the total mass profile that varies with the stellar half-light radius. We also discuss constraints on the baryon fraction, super-massive black holes, and axial ratio of the dark matter halo. Finally, we review constraints on non-thermal gas motions and discuss the accuracy of the hydrostatic equilibrium approximation in elliptical galaxies.

astro-ph.CO↗

arXiv2011

Detection of A Transient X-ray Absorption Line Intrinsic to the BL Lacertae Object H 2356-309

Since the launch of the Einstein X-ray Observatory in the 1970s, a number of broad absorption features have been reported in the X-ray spectra of BL Lac objects. These features are often interpreted as arising from high velocity outflows intrinsic to the BL Lac object, therefore providing important information about the inner environment around the central engine. However, such absorption features have not been observed more recently with high-resolution X-ray telescopes such as Chandra and XMM-Newton. In this paper, we report the detection of a transient X-ray absorption feature intrinsic to the BL Lac object H 2356-309 with the Chandra X-ray Telescope. This BL Lac object was observed during XMM cycle 7, Chandra cycle 8 and 10, as part of our campaign to investigate X-ray absorption produced by the warm-hot intergalactic medium (WHIM) residing in the foreground large scale superstructure. During one of the 80 ksec, Chandra cycle 10 observations, a transient absorption feature was detected at 3.3-sigma (or 99.9% confidence level, accounting for the number of trials), which we identify as the OVIII K-alpha line produced by an absorber intrinsic to the BL Lac object. None of the other 11 observations showed this line. We constrain the ionization parameter (25 <~ Xi <~ 40) and temperature (10^5 < T < 2.5 10^7 K) of the absorber. This absorber is likely produced by an outflow with a velocity up to 1,500 km/s. There is a suggestion of possible excess emission on the long-wavelength side of the absorption line; however, the derived properties of the emission material are very different from those of the absorption material, implying it is unlikely a typical P Cygni-type profile.

astro-ph.HE↗

arXiv2011

A Census of Baryons and Dark Matter in an Isolated, Milky Way-sized Elliptical Galaxy

We present a study of the dark and luminous matter in the isolated elliptical galaxy NGC720, based on deep X-ray observations made with Chandra and Suzaku. The gas is reliably measured to ~R2500, allowing us to place good constraints on the enclosed mass and baryon fraction (fb) within this radius (M2500=1.6e12+/-0.2e12 Msun, fb(2500)=0.10+/-0.01; systematic errors are <~20%). The data indicate that the hot gas is close to hydrostatic, which is supported by good agreement with a kinematical analysis of the dwarf satellite galaxies. We confirm a dark matter (DM) halo at ~20-sigma. Assuming an NFW DM profile, our physical model for the gas distribution enables us to obtain meaningful constraints at scales larger than R2500, revealing that most of the baryons are in the hot gas. We find that fb within Rvir is consistent with the Cosmological value, confirming theoretical predictions that a ~Milky Way-mass (Mvir=3.1e12+/-0.4e12 Msun) galaxy can sustain a massive, quasi-hydrostatic gas halo. While fb is higher than the cold baryon fraction typically measured in similar-mass spiral galaxies, both the gas fraction (fg) and fb in NGC720 are consistent with an extrapolation of the trends with mass seen in massive galaxy groups and clusters. After correcting for fg, the entropy profile is close to the self-similar prediction of gravitational structure formation simulations, as observed in galaxy clusters. Finally, we find a strong heavy metal abundance gradient in the ISM similar to those observed in massive galaxy groups.

astro-ph.CO↗

arXiv2010

Confirmation of X-Ray Absorption by WHIM in the Sculptor Wall

In a previous paper we reported a 3-sigma detection of an absorption line from the Warm-Hot Intergalactic Medium (WHIM) using the Chandra and XMM X-ray grating spectra of the blazar H2356-309, the sight-line of which intercepts the Sculptor Wall, a large-scale superstructure of galaxies at z ~ 0.03. To verify our initial detection, we obtained a deep (500 ks), follow-up exposure of H2356-309 as part of the Cycle-10 Chandra Large Project Program. From a joint analysis of the Cycle-10 and previous (Cycle-8) Chandra grating data we detect the redshifted OVII WHIM line at a significance level of 3.4-sigma, a substantial improvement over the 1.7-sigma level reported previously when using only the Cycle-8 data. The significance increases to 4.0-sigma when the existing XMM grating data are included in the analysis, thus confirming at higher significance the existence of the line at the redshift of the Sculptor Wall with an equivalent width of 28.5+/-10.5 mA (90% confidence). We obtain a 90% lower limit on the OVII column density of 0.8 10^16 cm^-2 and a 90% upper limit on the Doppler-b parameter of 460 km/s. Assuming the absorber is uniformly distributed throughout the ~ 15 Mpc portion of the blazar's sight-line that intercepts the Sculptor Wall, that the OVII column density is ~ 2 10^16 cm^-2 (corresponding to b > 150 km/s where the inferred column density is only weakly dependent on b), and that the oxygen abundance is 0.1 solar, we estimate a baryon over-density of ~ 30 for the WHIM, which is consistent with the peak of the WHIM mass fraction predicted by cosmological simulations. The clear detection of OVII absorption in the Sculptor Wall demonstrates the viability of using current observatories to study WHIM in the X-ray absorption spectra of blazars behind known large-scale structures.

astro-ph.CO↗

arXiv2010

The slope of the mass profile and the tilt of the fundamental plane in early-type galaxies

We present a survey, using the Chandra X-ray observatory, of the central gravitating mass profiles in a sample of 10 galaxies, groups and clusters, spanning ~2 orders of magnitude in virial mass. We find the total mass distributions from ~0.2--10Re, where Re is the optical effective radius of the central galaxy, are remarkably similar to powerlaw density profiles. The negative logarithmic slope of the mass density profiles, alpha, systematically varies with Re, from alpha=2, for systems with Re~4kpc to alpha=1.2 for systems with Re>30kpc. Departures from hydrostatic equilibrium are likely to be small and cannot easily explain this trend. We show that the conspiracy between the baryonic (Sersic) and dark matter (NFW/ Einasto) components required to maintain a powerlaw total mass distribution naturally predicts an anti-correlation between alpha and Re that is very close to what is observed. The systematic variation of alpha with Re implies a dark matter fraction within Re that varies systematically with the properties of the galaxy in such a manner as to reproduce, without fine tuning, the observed tilt of the fundamental plane. We speculate that establishing a nearly powerlaw total mass distribution is therefore a fundamental feature of galaxy formation and the primary factor which determines the tilt of the fundamental plane.

astro-ph.CO↗

arXiv2009

X-ray Isophotes in a Rapidly Rotating Elliptical Galaxy: Evidence of Inflowing Gas

We describe two-dimensional gasdynamical computations of the X-ray emitting gas in the rotating elliptical galaxy NGC 4649 that indicate an inflow of about one solar mass per year at every radius. Such a large instantaneous inflow cannot have persisted over a Hubble time. The central constant-entropy temperature peak recently observed in the innermost 150 parsecs is explained by compressive heating as gas flows toward the central massive black hole. Since the cooling time of this gas is only a few million years, NGC 4649 provides the most acutely concentrated known example of the cooling flow problem in which the time-integrated apparent mass that has flowed into the galactic core exceeds the total mass observed there. This paradox can be resolved by intermittent outflows of energy or mass driven by accretion energy released near the black hole. Inflowing gas is also required at intermediate kpc radii to explain the ellipticity of X-ray isophotes due to spin-up by mass ejected by stars that rotate with the galaxy and to explain local density and temperature profiles. We provide evidence that many luminous elliptical galaxies undergo similar inflow spin-up. A small turbulent viscosity is required in NGC 4649 to avoid forming large X-ray luminous disks that are not observed, but the turbulent pressure is small and does not interfere with mass determinations that assume hydrostatic equilibrium.

astro-ph.CO↗

arXiv2009

Hydrostatic Gas Constraints on Supermassive Black Hole Masses: Implications for Hydrostatic Equilibrium and Dynamical Modelling in a Sample of Early-Type Galaxies

We present new mass measurements for the supermassive black holes (SMBHs) in the centres of three early-type galaxies. The gas pressure in the surrounding, hot interstellar medium (ISM) is measured through spatially resolved spectroscopy with the Chandra X-ray observatory, allowing the SMBH mass (Mbh) to be inferred directly under the hydrostatic approximation. This technique does not require calibration against other SMBH measurement methods and its accuracy depends only on the ISM being close to hydrostatic, which is supported by the smooth X-ray isophotes of the galaxies. Combined with results from our recent study of the elliptical galaxy NGC4649, this brings to four the number of galaxies with SMBHs measured in this way. Of these, three already have mass determinations from the kinematics of either the stars or a central gas disc, and hence join only a handful of galaxies with Mbh measured by more than one technique. We find good agreement between the different methods, providing support for the assumptions implicit in both the hydrostatic and the dynamical models. The stellar mass-to-light ratios for each galaxy inferred by our technique are in agreement with the predictions of stellar population synthesis models assuming a Kroupa initial mass function (IMF). This concurrence implies that no more than ~10-20% of the ISM pressure is nonthermal, unless there is a conspiracy between the shape of the IMF and nonthermal pressure. Finally, we compute Bondi accretion rates, finding that the two galaxies with the highest rates exhibit little evidence of X-ray cavities, suggesting that the correlation with the AGN jet power takes time to be established.

astro-ph.CO↗

arXiv2009

X-Ray Absorption By WHIM in the Sculptor Wall

We present XMM RGS and Chandra LETG observations of the blazar, H 2356-309, located behind the Sculptor Wall, a large-scale galaxy structure expected to harbor high-density Warm-Hot Intergalactic Medium (WHIM). Our simultaneous analysis of the RGS and LETG spectra yields a 3-sigma detection of the crucial redshifted O vii K-alpha line with a column density (>~ 10^{16} cm^{-2}) consistent with similar large-scale structures produced in cosmological simulations. This represents the first detection of non-local WHIM from X-ray absorption studies where XMM and Chandra data are analyzed simultaneously and the absorber redshift is already known, thus providing robust evidence for the expected repository of the "missing baryons".

astro-ph.CO↗

arXiv2008

Chi-square and Poissonian Data: Biases Even in the High-Count Regime and How to Avoid them

We demonstrate that two approximations to the chi^2 statistic as popularly employed by observational astronomers for fitting Poisson-distributed data can give rise to intrinsically biased model parameter estimates, even in the high counts regime, unless care is taken over the parameterization of the problem. For a small number of problems, previous studies have shown that the fractional bias introduced by these approximations is often small when the counts are high. However, we show that for a broad class of problem, unless the number of data bins is far smaller than \sqrt{N_c}, where N_c is the total number of counts in the dataset, the bias will still likely be comparable to, or even exceed, the statistical error. Conversely, we find that fits using Cash's C-statistic give comparatively unbiased parameter estimates when the counts are high. Taking into account their well-known problems in the low count regime, we conclude that these approximate chi^2 methods should not routinely be used for fitting an arbitrary, parameterized model to Poisson-distributed data, irrespective of the number of counts per bin, and instead the C-statistic should be adopted. We discuss several practical aspects of using the C-statistic in modelling real data. We illustrate the bias for two specific problems, measuring the count-rate from a lightcurve and obtaining the temperature of a thermal plasma from its X-ray spectrum measured with the Chandra X-ray observatory. In the context of X-ray astronomy, we argue the bias could give rise to systematically mis-calibrated satellites and a ~5-10% shift in galaxy cluster scaling relations.

astro-ph↗

arXiv2008

Rotation and Turbulence of the Hot ICM in Galaxy Clusters

Cosmological simulations of galaxy clusters typically find that the weight of a cluster at a given radius is not balanced entirely by the thermal gas pressure of the hot ICM, with theoretical studies emphasizing the role of random turbulent motions to provide the necessary additional pressure support. Using a set of high-resolution, hydrodynamical simulations of galaxy clusters that include radiative cooling and star formation, we find instead that in the most relaxed clusters rotational support exceeds that from random turbulent motions for radii 0.1 - 0.5 r_500, and that the observed clusters are much rounder than the simulated, relaxed clusters within ~ 0.4 r_500. Moreover, while the observed clusters display an average ellipticity profile that does not vary significantly with radius, the ellipticity of the relaxed CDM clusters declines markedly with increasing radius, suggesting that the ICM of the observed clusters rotates less rapidly than that of the relaxed CDM clusters out to ~ 0.6 r_500. We also find the ellipticity profile of a simulated cluster without radiative cooling is in much better agreement with the observations, implying that over-cooling has a substantial impact on the gas dynamics and morphology out to larger radii than previously recognized. It also suggests that the 10%-20% systematic errors from non-thermal gas pressure support reported for simulated cluster masses, obtained from fitting simulated X-ray data over large radial ranges within r_500, may need to be revised downward. These results demonstrate the utility of X-ray ellipticity profiles as a probe of ICM rotation and over-cooling which should be used to constrain future cosmological cluster simulations.

astro-ph↗

arXiv2008

Low-Mass X-ray Binaries and Globular Clusters in Early-Type Galaxies. I. Chandra Observations

We present a Chandra survey of LMXBs in 24 early-type galaxies. Correcting for detection incompleteness, the X-ray luminosity function (XLF) of each galaxy is consistent with a powerlaw with negative logarithmic differential slope, beta~2.0. However, beta strongly correlates with incompleteness, indicating the XLF flattens at low-Lx. The composite XLF is well-fitted by a powerlaw with a break at 2.21(+0.65,-0.56)E38 erg/s and beta=1.40(+0.10,-0.13) and 2.84(+0.39,-0.30) below and above it, respectively. The break is close to the Eddington limit for a 1.4Msun neutron-star, but the XLF shape rules out its representing the division between neutron-star and black-hole systems. Although the XLFs are similar, we find evidence of some variation between galaxies. The high-Lx XLF slope does not correlate with age, but may correlate with [alpha/Fe]. Considering only LMXBs with Lx>1E37 erg/s, matching the LMXBs with globular clusters (GCs) identified in HST observations of 19 of the galaxies, we find the probability a GC hosts an LMXB is proportional to LGC^alpha ZFe^gamma} where alpha=1.01+/-0.19 and gamma=0.33+/-0.11. Correcting for GC luminosity and colour effects, and detection incompleteness, we find no evidence that the fraction of LMXBs with Lx>1e37 erg/s in GCs (40%), or the fraction of GCs hosting LMXBs (~6.5%) varies between galaxies. The spatial distribution of LMXBs resembles that of GCs, and the specific frequency of LMXBs is proportional to the GC specific luminosity, consistent with the hypothesis that all LMXBs form in GCs. If the LMXB lifetime is tau and the duty cycle is Fd, our results imply ~1.5 (tau/1E8 yr)^-1 /Fd LMXBs are formed per Gyr per GC and we place an upper limit of 1 active LMXB in the field per 3.4E9Lsun of V-band luminosity.

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