A. G. Kuranov

On the nature of the break in the X-ray luminosity function of low-mass X-ray binaries

We analyze a flux-limited sample of persistent and bright (with 2–10 keV fluxes exceeding 1.4 × 10 −10 erg s −1 cm −2 ) low-mass X-ray binaries (LMXBs) in our Galaxy. It is demonstrated that the majority of binary systems with X-ray luminosities below logL(erg/s) ∼ 37.3 have unevolved secondary companions (except for those with white dwarf donors), while systems with higher X-ray luminosity harbor predominantly giant donors. Mass transfer in binary systems with giants significantly shortens their lifetime thus steepening the X-ray luminosity function of LMXBs at high luminosity. We argue that this is why the LMXB luminosity function constructed from observations of sources in our and distant galaxies demonstrates a break at logL(erg/s) ∼ 37.3.

Population synthesis for symbiotic X-ray binaries

Symbiotic X-ray binaries (SyXBs) comprise a rare class of low-mass X-ray binaries. We study the Galactic SyXBs, which we consider as detached binaries composed of low-mass giants and wind-fed neutron star (NS) companions, by simulation of the interaction of a magnetized NS with its environment and utilizing a population synthesis code. We focus mainly on the parameters that influence the observational appearance of an SyXB: the donor wind velocity (vw) and the angular momentum distribution in the shell of matter settling on to an NS. We estimate the birthrate of SyXBs as similar to 4.1 x 10-5 to similar to 6.6 x 10-6?yr-1 and their number in the Galaxy as similar to 1001000. The assumed stellar wind velocity from cool giants is the input parameter that influences the model SyXB population most. Among known SyXBs or candidate systems, 4U 1954+31 and IGR J16358-4724 in which the NSs have very long spin periods may host quasi-spherically accreting NSs. GX 1+4 has a peculiar long-term spin behaviour and it may also be a quasi-spherical wind-accreting source. We cannot identify whether there are wind-fed accretion discs in 4U 1700+24, Sct X-1, IRXS J180431.1-273932 and 2XMM J174016.0-290337.

The luminosity function of low-mass X-ray binaries in galaxies

X-ray luminosity function derived from observations of X-ray sources in galactic bulges can be explained by prinicipal evolutionary relations for mass accretion rate onto the compact object. The observed mean distribution of individual X-ray luminosities of galactic LMXB is satisfactorily described by a symmetric quasi-Lorentzian curve. The flux variance for bright sources is found to be proportional to the mean luminosity. Such a distribution does not change the slope of the power law luminosity function of the source population, which is expected from the dependence of the mass transfer rate on the mass of the Roche-lobe filling non-degenerate optical component.The X-ray luminosity function of low-mass binaries constructed from the observations of point like X-ray sources in galactic bulges can be explained in terms of the main evolutionary relations for the rate of mass transfer onto a compact object. The observed scatter of luminosities for individual low-mass X-ray sources in our Galaxy is shown to be satisfactorily described by a symmetric quasi-Lorentz curve with a dispersion proportional to the mean luminosity. Such a form of the mean luminosity function for individual sources does not affect the power-law pattern of the luminosity function for the entire population of sources that is expected for a power-law dependence of the mass transfer rate in a close binary on the mass of the Roche lobe—filling optical component.

Pulsar spin–velocity alignment from single and binary neutron star progenitors

The role of binary progenitors of neutron stars (NSs) in the apparent distribution of space velocities and spin-velocity alignment observed in young pulsars is studied. We performed a Monte Carlo synthesis of pulsar populations originated from single and binary stars with different assumptions about the NS natal kick (kick―spin alignment, kick amplitude and kick reduction in electron-capture supernovae in binary progenitors with initial main-sequence masses from the range 8-11 M ⊙ which experienced mass exchange due to Roche lobe overflow). The calculated spin-velocity alignment in pulsars is compared with data inferred from radio polarization measurements. The observed space velocity of pulsars is found to be mostly affected by the natal kick velocity form and its amplitude; the fraction of binaries is not important here for reasonably large kicks. The natal kick―spin alignment is found to strongly affect the spin-velocity correlation of pulsars. Comparison with the observed pulsar spin―velocity angles favours a sizeable fraction of binary progenitors and kick-spin angles ∼5°―20°.

NEUTRON STARS IN GLOBULAR CLUSTERS: FORMATION AND OBSERVATIONAL MANIFESTATIONS

Population synthesis is used to model the number of neutron stars in globular clusters that are observed as low-mass X-ray sources and millisecond radio pulsars. The dynamical interactions between binary and single stars in a cluster are assumed to take place only with a continuously replenished “background” of single stars whose properties keep track of the variations in parameters of the cluster as a whole and the evolution of single stars. We use the hypothesis that the neutron stars forming in binary systems from components with initial masses of ∼8–12 M⊙ during the collapse of degenerate O-Ne-Mg cores through electron captures do not acquire a high space velocity. The remaining neutron stars (from single stars with masses >8 M⊙ or from binary components with masses >12 M⊙) are assumed to be born with high space velocities. According to this hypothesis, a sizeable fraction of the forming neutron stars remain in globular clusters (about 1000 stars in a cluster with a mass of 5 × 105M⊙). The number of millisecond radio pulsars forming in such a cluster in the case of accretion-driven spinup in binary systems is found to be ∼10, in agreement with observations. Our modeling also reproduces the observed shape of the X-ray luminosity function for accreting neutron stars in binary systems with normal and degenerate components and the distribution of spin periods for millisecond pulsars.

On the dynamical formation of accreting intermediate mass black holes

ABSTRACT We compute the probability that intermediate mass black holes (IMBHs) cap-ture companions due to dynamical interactions and become accreting sources,and explore the possibility that the accreting IMBHs would appear as ultra-luminous X-ray sources (ULXs). We focus on IMBHs originating from low-metallicity Population III stars. Two channels of IMBH formation are consid-ered: from primordial halos in the framework of hierarchical clustering, andfrom non-mixed, zero-metallicity primeval gas in galactic discs. IMBHs canform binary systems due to tidal captures of single stars and exchange inter-actions with existing binary systems in galactic discs. We find that neitherformation mechanism of the accreting IMBH binary is able to provide enoughsources to explain the observed population of ULXs. Even at sub-ULX lu-minosity, the total number of accreting IMBHs with L > 10 36 erg s −1 withdynamically captured companions is found to be < 0.01 per galaxy.Key words: accretion, accretion discs - black hole physics, stars: formation- stars: evolution, galaxies: formation, X-rays: binaries

Neutron star spin–kick velocity correlation effect on binary neutron star coalescence rates and spin–orbit misalignment of the components

We study the effect of the neutron star spin-kick velocity alignment observed in young radio pulsars on the coalescence rate of binary neutron stars. Two scenarios are considered for neutron star formation: when the kick is always present, and when it is small or absent if a neutron star is formed in a binary system as a result of electron-capture degenerate core collapse. The effect is shown to be especially strong for large kick amplitudes and tight alignments, reducing the expected galactic rate of binary neutron star coalescence compared to calculations with randomly directed kicks. The spin-kick correlation also leads to a much narrower neutron star spin-orbit misalignment.

Modeling the luminosity function of galactic low-mass X-ray binaries

The evolution of the family of binaries with a low-mass star and a compact neutron star companion (low-mass X-ray binaries (LMXBs) with neutron stars) ismodeled by the method of population synthesis. Continuous Roche-lobe filling by the optical star in LMXBs is assumed to be maintained by the removal of orbital angular momentum from the binary by a magnetic stellar wind from the optical star and the radiation of gravitational waves by the binary. The developed model of LMXB evolution has the following significant distinctions: (1) allowance for the effect of the rotational evolution of a magnetized compact remnant on themass transfer scenario in the binary, (2) amore accurate allowance for the response of the donor star to mass loss at the Roche-lobe filling stage. The results of theoretical calculations are shown to be in good agreement with the observed orbital period-X-ray luminosity diagrams for persistent Galactic LMXBs and their X-ray luminosity function. This suggests that the main elements of binary evolution, on the whole, are correctly reflected in the developed code. It is shown that most of the Galactic bulge LMXBs at luminosities Lx > 1037 erg s−1 should have a post-main-sequence Roche-lobe-filling secondary component (low-mass giants). Almost all of the models considered predict a deficit of LMXBs at X-ray luminosities near ∼1036.5 erg s−1 due to the transition of the binary from the regime of angular momentum removal by a magnetic stellar wind to the regime of gravitational waves (analogous to the widely known period gap in cataclysmic variables, accreting white dwarfs). At low luminosities, the shape of the model luminosity function for LMXBs is affected significantly by their transient behavior-the accretion rate onto the compact companion is not always equal to the mass transfer rate due to instabilities in the accretion disk around the compact object. The best agreement with observed binaries is achieved in the models suggesting that heavy neutron stars with masses 1.4–1.9M⊙ can be born.

Broadband gravitational-wave pulses from binary neutron stars in eccentric orbits

AbstractThe gravitational-wave radiation from binary stars in elliptical orbits peaks at times close to the periastron passage. For a stationary distribution of binary neutron stars in the Galaxy, there are several systems with large orbital eccentricities and periods in the range from several tens of minutes to several days from which gravitational-wave radiation at periastron will be observed as a broad pulse in the frequency range 1–100 mHz. The LISA space interferometer will be able to record pulsed signals from these systems at a signal-to-noise ratio