arXiv subjects

V. Kalogera

Publications and source records attributed to V. Kalogera.

At least 55 records · Page 3Linked to original sources

arXiv2007

The Dearth of low-luminosity Globular Cluster LMXBs in NGC 3379

Our campaign of deep monitoring observations with {\it Chandra} of the nearby elliptical galaxy NGC 3379 has lead to the detection of nine globular cluster (GC) and 53 field low mass X-ray binaries (LMXBs) in the joint {\it Hubble}/{\it Chandra} field of view of this galaxy. Comparing these populations, we find a highly significant lack of GC LMXBs at the low (0.3-8 keV) X-ray luminosities (in the $\sim 10^{36}$ to $\sim 4\times10^{37}$ erg s$^{-1}$ range) probed with our observations. This result conflicts with the proposition that all LMXBs are formed in GCs. This lack of low-luminosity sources in GCs is consistent with continuous LMXB formation due to stellar interactions and with the transition from persistent to transient LMXBs. The observed cut-off X-ray luminosity favors a predominance of LMXBs with main-sequence donors instead of ultra-compact binaries with white-dwarf donors; ultra-compacts could contribute significantly only if their disks are not affected by X-ray irradiation. Our results suggest that current theories of magnetic stellar wind braking may work rather better for the unevolved companions of GC LMXBs than for field LMXBs and cataclysmic variables in the Galaxy, where these companions may be somewhat evolved.

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arXiv2007

Polar kicks and the spin period - eccentricity relation in double neutron stars

We present results of a population synthesis study aimed at examining the role of spin-kick alignment in producing a correlation between the spin period of the first-born neutron star and the orbital eccentricity of observed double neutron star binaries in the Galactic disk. We find spin-kick alignment to be compatible with the observed correlation, but not to alleviate the requirements for low kick velocities suggested in previous population synthesis studies. Our results furthermore suggest low- and high-eccentricity systems may form through two distinct formation channels distinguished by the presence or absence of a stable mass transfer phase before the formation of the second neutron star. The presence of highly eccentric systems in the observed sample of double neutron stars may furthermore support the notion that neutron stars accrete matter when moving through the envelope of a giant companion.

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arXiv2007

Constraining population synthesis models via empirical binary compact object merger and supernovae rates

The observed samples of supernovae (SN) and double compact objects (DCOs) provide several critical constraints on population-synthesis models: the parameters of these models must be carefully chosen to reproduce, among other factors, (i) the formation rates of double neutron star (NS-NS) binaries and of white dwarf-neutron star (WD-NS) binaries, estimated from binary samples, and (ii) the type II and Ib/c supernova rates. Even allowing for extremely conservative accounting of the uncertainties in observational and theoretical predictions, we find only a few plausible population synthesis models (roughly 9%) are consistent with DCO and SN rates empirically determined from observations. As a proof of concept, we describe the information that can be extracted about population synthesis models given such stringent observational tests, including surprisingly good agreement with the neutron star kick distributions inferred from pulsar proper-motion measurements. In the present study, we find that the current observational constraints favor: kicks described by a single Maxwellian with a typical velocity of about 300km/s; mass-loss fractions during non-conservative, but stable, mass transfer episodes of about 90%; and common envelope parameters of about 0.2-0.5. Finally, we use the subset of astrophysically consistent models to predict the rates at which black hole-neutron star (BH-NS) and NS-NS binaries merge in the Milky Way and the nearby Universe, assuming Milky-Way-like galaxies dominate. (Abridged)

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arXiv2007

Interacting Binaries with Eccentric Orbits. Secular Orbital Evolution Due To Conservative Mass Transfer

We investigate the secular evolution of the orbital semi-major axis and eccentricity due to mass transfer in eccentric binaries, assuming conservation of total system mass and orbital angular momentum. Assuming a delta function mass transfer rate centered at periastron, we find rates of secular change of the orbital semi-major axis and eccentricity which are linearly proportional to the magnitude of the mass transfer rate at periastron. The rates can be positive as well as negative, so that the semi-major axis and eccentricity can increase as well as decrease in time. Adopting a delta-function mass-transfer rate of $10^{-9} M_\sun {\rm yr}^{-1}$ at periastron yields orbital evolution timescales ranging from a few Myr to a Hubble time or more, depending on the binary mass ratio and orbital eccentricity. Comparison with orbital evolution timescales due to dissipative tides furthermore shows that tides cannot, in all cases, circularize the orbit rapidly enough to justify the often adopted assumption of instantaneous circularization at the onset of mass transfer. The formalism presented can be incorporated in binary evolution and population synthesis codes to create a self-consistent treatment of mass transfer in eccentric binaries.

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arXiv2007

Compact Object Modeling with the StarTrack Population Synthesis Code

We present a comprehensive description of the population synthesis code StarTrack. The original code has been significantly modified and updated. Special emphasis is placed here on processes leading to the formation and further evolution of compact objects (white dwarfs, neutron stars, and black holes). Both single and binary star populations are considered. The code now incorporates detailed calculations of all mass-transfer phases, a full implementation of orbital evolution due to tides, as well as the most recent estimates of magnetic braking. This updated version of StarTrack can be used for a wide variety of problems, with relevance to many current and planned observatories, e.g., studies of X-ray binaries (Chandra, XMM-Newton), gravitational radiation sources (LIGO, LISA), and gamma-ray burst progenitors (HETE-II, Swift). The code has already been used in studies of Galactic and extra-galactic X-ray binary populations, black holes in young star clusters, Type Ia supernova progenitors, and double compact object populations. Here we describe in detail the input physics, we present the code calibration and tests, and we outline our current studies in the context of X-ray binary populations.

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arXiv2007

Mapping population synthesis event rates on model parameters II: Convergence and accuracy of multidimensional fits

Binary population synthesis calculations and associated predictions, especially event rates, are known to depend on a significant number of input model parameters with different degrees of sensitivity. At the same time, for systems with relatively low formation rates, such simulations are heavily computationally demanding and therefore the needed explorations of the high-dimensional parameter space require major -- often prohibitive -- computational resources. In the present study, to better understand several key event rates involving binary evolution and binaries with two compact objects in Milky Way-like galaxies and to provide ways of reducing the computational costs of complete parameter space explorations: (i) we perform a methodical parameter study of the \emph{StarTrack} population synthesis code ; and (ii) we develop a formalism and methodology for the derivation of {\em multi-dimensional fits} for event rates. We significantly generalize our earlier study, and we focus on ways of thoroughly assessing the accuracy of the fits. We anticipate that the efficient tools developed here can be applied in lieu of large-scale population calculations and will facilitate the exploration of the dependence of rate predictions on a wide range binary evolution parameters. Such explorations can then allow the derivation of constraints on these parameters, given empirical rate constraints and accounting for fitting errors. Here we describe in detail the principles and practice behind constructing these fits, estimating their accuracy, and comparing them with observations in a manner that accounts for their errors.

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arXiv2007

Equipotential Surfaces and Lagrangian points in Non-synchronous, Eccentric Binary and Planetary Systems

We investigate the existence and properties of equipotential surfaces and Lagrangian points in non-synchronous, eccentric binary star and planetary systems under the assumption of quasi-static equilibrium. We adopt a binary potential that accounts for non-synchronous rotation and eccentric orbits, and calculate the positions of the Lagrangian points as functions of the mass ratio, the degree of asynchronism, the orbital eccentricity, and the position of the stars or planets in their relative orbit. We find that the geometry of the equipotential surfaces may facilitate non-conservative mass transfer in non-synchronous, eccentric binary star and planetary systems, especially if the component stars or planets are rotating super-synchronously at the periastron of their relative orbit. We also calculate the volume-equivalent radius of the Roche lobe as a function of the four parameters mentioned above. Contrary to common practice, we find that replacing the radius of a circular orbit in the fitting formula of Eggleton (1983) with the instantaneous distance between the components of eccentric binary or planetary systems does not always lead to a good approximation to the volume-equivalent radius of the Roche-lobe. We therefore provide generalized analytic fitting formulae for the volume-equivalent Roche lobe radius appropriate for non-synchronous, eccentric binary star and planetary systems. These formulae are accurate to better than 1% throughout the relevant 2-dimensional parameter space that covers a dynamic range of 16 and 6 orders of magnitude in the two dimensions.

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arXiv2006

Formation of Double Compact Objects

Current observations of double neutron stars provide us with a wealth of information that we can use to investigate their evolutionary history and the physical conditions of neutron star formation. Understanding this history and formation conditions further allow us to make theoretical predictions for the formation of other double compact objects with one or two black hole components and assess the detectability of such systems by ground-based gravitational-wave interferometers. In this paper we summarize our group's body of work in the past few years and we place our conclusions and current understanding in the framework of other work in this area of astrophysical research.

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arXiv2006

Black Hole Formation in X-Ray Binaries: The Case of XTE J1118+480

In recent years, an increasing number of proper motions have been measured for Galactic X-ray binaries. When supplemented with accurate determinations of the component masses, orbital period, and donor luminosity and effective temperature, these kinematical constraints harbor a wealth of information on the systems' past evolution. The constraints on compact object progenitors and kicks derived from this are of immense value for understanding compact object formation and exposing common threads and fundamental differences between black hole and neutron star formation. Here, we present the results of such an analysis for the black hole X-ray binary XTE J1118+480. We present results from modeling the mass transfer phase, following the motion in the Galaxy back to the birth site of the black hole, and examining the dynamics of symmetric and asymmetric core-collapses of the black hole progenitor.

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arXiv2006

On the Formation and Progenitor of PSR J0737-3039: New Constraints on the Supernova Explosion Forming Pulsar B

We revisit the formation of PSR J0737-3039, taking into account the most recent observational constraints. We show that the most likely kick velocity and progenitor parameters depend strongly on the consideration of the full five-dimensional PDF for the magnitude and direction of the kick velocity imparted to pulsar B at birth, the mass of pulsar B's pre-supernova helium star progenitor, and the pre-supernova orbital separation, and on the adopted prior assumptions. The priors consist of the transverse systemic velocity, the age of the system, and the treatment of the unknown radial velocity. Since the latter cannot be determined from observation, we adopt a statistical approach and use theoretical radial-velocity distributions obtained from population synthesis calculations for coalescing double neutron stars. We find that the prior assumptions about the pre-supernova helium star mass affect the derived most likely parameters significantly: when the minimum helium star mass required for neutron star formation is assumed to be 2.1Msun, the most likely kick velocity ranges from 70-180km/s; when masses lower than 2.1Msun are assumed to allow neutron star formation, the most likely kick velocity can be as low as a few km/s, although the majority of the considered models still yield most likely kick velocities of 50-170km/s. We also show that the proximity of the double pulsar to the Galactic plane and the small proper motion do not pose stringent constraints on the kick velocity and progenitor mass of pulsar B. Instead, the constraints imposed by the orbital dynamics of asymmetric supernova explosions turn out to be much more restrictive. We conclude that the currently available observational constraints cannot be used to favor a specific core-collapse and neutron star formation mechanism. (abridged)

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arXiv2006

Probing the Low-Luminosity XLF in Normal Elliptical Galaxies

We present the first low luminosity (LX > 5 - 10 1036 erg s-1) X-ray luminosity functions (XLFs) of low-mass X-ray binaries (LMXBs) determined for two typical old elliptical galaxies, NGC 3379 and NGC 4278. Because both galaxies contain little diffuse emission from hot ISM and no recent significant star formation (hence no high-mass X-ray binary contamination), they provide two of the best homogeneous sample of LMXBs. With 110 and 140 ks Chandra ACIS S3 exposures, we detect 59 and 112 LMXBs within the D25 ellipse of NGC 3379 and NGC 4278, respectively. The resulting XLFs are well represented by a single power-law with a slope (in a differential form) of 1.9 0.1. In NGC 4278, we can exclude the break at LX ~ 5 x 1037 erg s-1 that was recently suggested to be a general feature of LMXB XLFs. In NGC 3379 instead we find a localized excess over the power law XLF at ~4 x 1037 erg s-1, but with a marginal significance of ~1.6s. Because of the small number of luminous sources, we cannot constrain the high luminosity break (at 5 x 1038 erg s-1) found in a large sample of early type galaxies. While the optical luminosities of the two galaxies are similar, their integrated LMXB X-ray luminosities differ by a factor of 4, consistent with the relation between the X-ray to optical luminosity ratio and the globular cluster specific frequency.

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arXiv2006

The Eclipsing ULX in NGC 3379

We report recent Chandra observations of the ULX in the elliptical galaxy NGC3379 that clearly detect two flux variability cycles. Comparing these data with the Chandra observation of ~5yr ago, we measure a flux modulation with a period of ~12.6hr. Moreover, we find that the emission undergoes a correlated spectral modulation, becoming softer at low flux. We argue that our results establish this source as a ULX binary in NGC3379. Given the old stellar population of this galaxy, the ULX is likely to be a soft transient, however historical X-ray sampling suggests that the current on phase has lasted ~10yr. We discuss our results in terms of ADC and wind-feedback models. We constrain the donor mass and orbital period at the onset of mass transfer within 1.15-1.4M and 12.5-16hr, respectively. The duration of the mass-transfer phase so far is probably ~1Gyr and the binary has been a soft X-ray transient throughout this time. These constraints are insensitive to the mass of the accretor.

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arXiv2005

The Brightest Point X-Ray Sources in Elliptical Galaxies and the Mass Spectrum of Accreting Black Holes

We propose that the shape of the upper-end X-ray luminosity function observed in elliptical galaxies for point sources is a footprint of the black-hole (BH) mass spectrum among old X-ray transients formed in the galaxies. We show that this underlying BH mass spectrum is modified by a weighting factor that is related to the transient duty cycle and it generally depends on the BH mass and XRB donor type (main-sequence, red-giant, or white-dwarf donors). A duty cycle that depends on the binary mass-transfer rate relative to the critical rate for transient behavior is most probably favored. We also find that the derived BH mass spectrum slope depends on the strength of angular momentum loss due to magnetic braking for main-sequence donors. More specifically, we find that, for ``standard'' magnetic braking, BH XRBs with red-giant donors dominate the upper-end XLF; for weaker magnetic braking prescriptions main-sequence donors are found to be dominant. In both cases the BH mass spectrum has a differential slope of ~2.5 and an upper BH mass cut-off at ~20 Msun is needed to understand the very brightest of the BH XRBs in elliptical galaxies. We expect that our result will help to constrain binary population synthesis models and the adopted relations between black holes and the masses of their progenitors.

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arXiv2005

Close Binary Interactions of Intermediate-Mass Black Holes: Possible Ultra-Luminous X-ray Sources?

While many observed ultra-luminous X-ray sources (ULXs, Lx > 10^39 erg s^-1) could be extragalactic X-ray binaries (XRBs) emitting close to the Eddington limit, the highest-luminosity ULXs (Lx > 3x10^39 erg s^-1) exceed the isotropic Eddington luminosity for even high-stellar-mass accreting black hole XRBs. It has been suggested that these highest-luminosity ULXs may contain accreting intermediate-mass black hole (IMBH) binaries. We consider this hypothesis for dense, young (about 100 Myr) stellar clusters where we assume that a 50-500 solar mass central IMBH has formed through runaway growth of a massive star. Using numerical simulations of the dynamics and evolution of the central black hole's captured companions, we obtain estimates of the incidence of mass transfer phases and possible ULX activity throughout the IMBH's evolutionary history. We find that, although it is common for the central black hole to acquire binary companions, there is a very low probability that these interacting binaries will become observable ULX sources.

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arXiv2005

Mass transfer and the period decrease in RXJ0806.3+1527

We examine the nature of RXJ0806.3+1527 and show that it is possible to reconcile the observed period decrease and X-ray luminosity with the transfer of mass between two white dwarfs provided that: either the system is (i) still in the early and short-lived (less than ~100yr) stages of mass transfer due to atmospheric Roche-lobe overflow, or (ii) in a standard, long-term, quasi-stationary mass-transfer phase that is significantly (~90%) non-conservative and the conversion of accretion energy to X-rays is quite inefficient. In either of the two cases and for a wide range of physical parameters, we find that orbital angular momentum is lost from the system at a rate that is a factor of a few (less than ~4) higher than the rate associated with the emission of gravitational waves. Although the physical origin of this extra angular momentum loss is not clear at present, it should be taken into account in the consideration of RXJ0806.3+1527 as a verification Galactic source for LISA.

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arXiv2005

Bounds on Expected Black Hole Spins in Inspiraling Binaries

As a first step towards understanding the angular momentum evolution history of black holes in merging black-hole/neutron-star binaries, we perform population synthesis calculations to track the distribution of accretion histories of compact objects in such binaries. We find that there are three distinct processes which can possibly contribute to the black-hole spin magnitude: a birth spin for the black hole, imparted at either (i) the collapse of a massive progenitor star to a black hole or (ii) the accretion-induced collapse of a neutron star to a black hole; and (iii) an accretion spin-up when the already formed black hole [via (i) or (ii)] goes through an accretion episode (through an accretion disk or a common-envelope phase). Our results show that, with regard to accretion-induced spinup in merging BH-NS binaries [method (iii) above], only {\em accretion episodes associated with common-envelope phases and hypercritical accretion rates} occur in the formation history of merging black hole/neutron star binaries. Lacking unambiguous experimental information about BH birth spins [i.e., regarding the results of processes (i) and (ii)], we choose two fiducial values for the BH birth angular momentum parameter a=J/M^2, consistent with observations of (i) NS birth spins (a roughly 0) and (ii) X-ray binaries (a=0.5). Using these two fiducial values and a conservative upper bound on the specific angular momentum of accreted matter, we discuss the expected range of black hole spins in the binaries of interest. We conclude with comments on the significance of these results for ground-based gravitational-wave searches of inspiral signals from black hole binaries.

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arXiv2005

Constraining population synthesis models via the binary neutron star population

The observed sample of double neutron-star (NS-NS) binaries presents a challenge to population-synthesis models of compact object formation: the parameters entering into these models must be carefully chosen so as to match (i) the observed star formation rate and (ii) the formation rate of NS-NS binaries, which can be estimated from the observed sample and the selection effects related to the discoveries with radio-pulsar surveys. In this paper, we select from an extremely broad family of possible population synthesis models those few (2%) which are consistent with the observed sample of NS-NS binaries. To further sharpen the constraints the observed NS-NS population places upon our understanding of compact-object formation processes, we separate the observed NS-NS population into two channels: (i) merging NS-NS binaries, which will inspiral and merge through the action of gravitational waves within $10 $ Gyr, and (ii) wide NS-NS binaries, consisting of all the rest. With the subset of astrophysically consistent models, we explore the implications for the rates at which double black hole (BH-BH), black hole-neutron star (BH-NS), and NS-NS binaries will merge through the emission of gravitational waves.

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arXiv2005

Are Supernova Kicks Responsible for X-ray Binary Ejection from Young Clusters?

Recent Chandra observations of interacting and starburst galaxies have led us to investigate the apparent correlation between the positions of young star clusters and Chandra point sources. Assumed to be X-ray binaries (XRBs), these point sources do not seem to coincide with the massive (~1e5 Msun), young (1-50 Myr) stellar clusters that can easily form systems capable of such emission. We use a sophisticated binary evolution and population synthesis code (StarTrack) and a simplified cluster model to track both the X-ray luminosity and position of XRBs as a function of time. These binaries are born within the cluster potential with self-consistent positions and velocities and we show that a large fraction (~70%) can be ejected from the parent due to supernova explosions and associated systemic velocities. For brighter sources and cluster masses below ~1e6 Msun, we find that the average number of bright XRBs per cluster remains near or below unity, consistent with current observations.

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