Neutron Stars In Close Binaries Research Articles

Formation of Fast-spinning Neutron Stars in Close Binaries and Magnetar-driven Stripped-envelope Supernovae

Formation of Fast-spinning Neutron Stars in Close Binaries and Magnetar-driven Stripped-envelope Supernovae

The Birth Function for Black Holes and Neutron Stars in Close Binaries

Abstract The mass function for black holes and neutron stars at birth is explored for mass-losing helium stars. These should resemble, more closely than similar studies of single hydrogen-rich stars, the results of evolution in close binary systems. The effects of varying the mass-loss rate and metallicity are calculated using a simple semi-analytic approach to stellar evolution that is tuned to reproduce detailed numerical calculations. Though the total fraction of black holes made in stellar collapse events varies considerably with metallicity, mass-loss rate, and mass cutoff, from 5% to 30%, the shapes of their birth functions are very similar for all reasonable variations in these quantities. Median neutron star masses are in the range 1.32–1.37 regardless of metallicity. The median black hole mass for solar metallicity is typically 8–9 if only initial helium cores below 40 (ZAMS mass less than 80 ) are counted, and 9–13 , in most cases, if helium cores with initial masses up to 150 (ZAMS mass less than 300 ) contribute. As long as the mass-loss rate as a function of mass exhibits no strong nonlinearities, the black hole birth function from 15 to 35 has a slope that depends mostly on the initial mass function for main-sequence stars. These findings imply the possibility of constraining the initial mass function and the properties of mass loss in close binaries using ongoing measurements of gravitational-wave radiation. The expected rotation rates of the black holes are briefly discussed.

Electron-positron energy deposition rate from neutrino pair annihilation in the equatorial plane of rapidly rotating neutron and quark stars

The neutrino–antineutrino annihilation into electron–positron pairs near the surface of compact general relativistic stars could play an important role in supernova explosions, neutron star collapse, or for close neutron star binaries near their last stable orbit. General relativistic effects increase the energy deposition rates due to the annihilation process. We investigate the deposition of energy and momentum due to the annihilations of neutrinos and antineutrinos in the equatorial plane of the rapidly rotating neutron and quark stars, respectively. We analyse the influence of general relativistic effects, and obtain the general relativistic corrections to the energy and momentum deposition rates for arbitrary stationary and axisymmetric space–times. We obtain the energy and momentum deposition rates for several classes of rapidly rotating neutron stars, described by different equations of state of the neutron matter, and for quark stars, described by the Massachusetts Institute of Technology bag model equation of state and in the colour-flavour-locked phase, respectively. Compared to the Newtonian calculations, rotation and general relativistic effects increase the total annihilation rate measured by an observer at infinity. The differences in the equations of state for neutron and quark matter also have important effects on the spatial distribution of the energy deposition rate by neutrino–antineutrino annihilation.

Neutron stars in close binaries with elliptical component orbits

We consider the evolution of close binaries in which the initial secondary component is a nondegenerate helium star with mass MHe = 0.4–60 M⊙, while the initially more massive primary has evolved into a black hole, neutron star, or degenerate dwarf. The neutron star is assumed to originate as a result of the evolution of a helium star with a mass of 2.5 M⊙ ≤ MHe ≤ 10 M⊙ after the explosion of a type Ib,c supernova. If the axial rotation of the helium star before the explosion is rigid-body and synchronized with the orbital rotation, for Porb ≤ 0.16 day, the rotational energy of the young neutron star will exceed the energy of an ordinary supernova. If the magnetic field of the neutron star is sufficiently strong, the necessary conditions for a magnetic-rotational supernova are provided. The initial rotational period of a young neutron star originating in a system with an orbital period shorter than ∼50 days is shorter than ∼4 s, which, according to observations, is required for the appearance of a radio pulsar. A helium star whose mass exceeds ∼10 M⊙ in a close binary with an orbital period shorter than one day and with the axial rotation of the helium presupernova synchronous with the orbital rotation evolves into a Kerr black hole, whose formation is likely to be accompanied by a gamma-ray burst with a duration longer than two seconds. In particular, we consider close binaries in which the second supernova results in the formation of a neutron star that remains in the binary. The theoretical distribution of orbital periods and eccentricities for such systems is consistent with that observed for radio pulsars in the Galactic disk in binaries with compact components and orbital eccentricities exceeding ∼0.09, providing an explanation for the observed correlation between the orbital eccentricities and orbital periods for these systems.

General relativistic models of binary neutron stars in quasiequilibrium

We perform fully relativistic calculations of binary neutron stars in corotating, circular orbit. While Newtonian gravity allows for a strict equilibrium, a relativistic binary system emits gravitational radiation, causing the system to lose energy and slowly spiral inwards. However, since inspiral occurs on a time scale much longer than the orbital period, we can treat the binary to be in quasiequilibrium. In this approximation, we integrate a subset of the Einstein equations coupled to the relativistic equation of hydrostatic equilibrium to solve the initial value problem for binaries of arbitrary separation. We adopt a polytropic equation of state to determine the structure and maximum mass of neutron stars in close binaries for polytropic indices $n=1,$ 1.5 and 2. We construct sequences of constant rest-mass and locate turning points along energy equilibrium curves to identify the onset of orbital instability. In particular, we locate the innermost stable circular orbit and its angular velocity. We construct the first contact binary systems in full general relativity. These arise whenever the equation of state is sufficiently soft $(n\ensuremath{\gtrsim}1.5).$ A radial stability analysis reveals no tendency for neutron stars in close binaries to collapse to black holes prior to merger.

Magnetic and spin evolution of neutron stars in close binaries

The evolution of neutron stars in close binary systems with a low-mass companion is considered, assuming the magnetic field to be confined within the solid crust. We adopt the standard scenario for the evolution in a close binary system, in which the neutron star passes through four evolutionary phases (‘isolated pulsar’–‘propeller’– accretion from the wind of a companion – accretion resulting from Roche-lobe overflow). Calculations have been performed for a great variety of parameters characterizing the properties of both the neutron star and the low-mass companion. We find that neutron stars with more or less standard magnetic field and spin period that are processed in low-mass binaries can evolve to low-field rapidly rotating pulsars. Even if the main-sequence life of a companion is as long as 1010 yr, the neutron star can maintain a relatively strong magnetic field to the end of the accretion phase. The model that is considered can account well for the origin of millisecond pulsars.

The Central Density of Neutron Stars in Close Binaries

Recent simulations of coalescing neutron stars conducted by Wilson, Mathews, and Marronetti show a rising central density as the orbital separation shrinks. Mathews and Wilson (MW) claim this ``star crushing'' is partially due to a nonlinear, first post-Newtonian order (1PN) enhancement of the self-gravity of each star caused by the other star. Here we show (contrary to the MW claim), within general relativity, 1PN interactions with the other star leave the central baryon density unchanged as the orbital radius shrinks. Alternative gravitational theories (e.g., Brans-Dicke) can exhibit a crushing of the stars as they near coalescence.

Spin and magnetic evolution of accreting neutron stars in close binaries

The paper considers the influence of magnetic field decay on the spin evolution of neutron stars in the course of an accretion phase. The simplest model is assumed for the interaction of the accreted matter with the neutron star magnetosphere. We analyse a few possible decay laws of the magnetic field which can be representative of various theoretical scenarios. For the considered decay laws, the neutron star entering a low-mass binary system can generally spin up to millisecond periods if it accretes 0.01-0.1 Mo. Our calculations show that a power-law decay of the magnetic field with the power P < 2 is most suitable for the evolution of 'standard' neutron stars with B ~ 1011 _1013 G to millisecond pulsars with B ~ 108_109 G.

Instabilities in Close Neutron Star Binaries.

We report on a new analysis of instabilities in close neutron star binaries based upon (3 + 1) dimensional general relativistic numerical hydrodynamics calculations. When a realistic equation of state is employed, orbit calculations for two $1.45{M}_{\ensuremath{\bigodot}}$ neutron stars reveal surprising evidence that general relativistic effects may cause otherwise stable stars to individually collapse prior to merging. Also, the strong fields cause the last stable orbit to occur at a larger separation distance and lower frequency than post-Newtonian estimates.

Physics of Neutron Stars

Since the first identification of neutron stars, in pulsars, a decade ago, theoretical and observational knowledge of these unusual objects has grown at a rapid rate. In this article we describe developments that have taken place since our 1975 review article on neutron stars (Baym & Pethick 1975, referred to hereafter as BP), as well as review several of their more astrophysical aspects not discussed there. The most striking observational fact about neutron stars is their existence: at present 321 pulsars, which are generally accepted to be rapidly rotating neutron stars, have been observed in our galaxy (Manchester et al. 1978, Taylor & Manchester 1977, Manchester & Taylor 1977, Smith 1976). In addition, most of the 16 pulsating compact X-ray sources so far discovered are likely to be accreting neutron stars in close binaries (for a review see Lamb 1977). The association of the Crab and Vela pulsars with supernova remnants provides evidence for the formation of neutron stars in supernovae, a picture supported to a limited extent by comparison of pulsar populations and lifetimes with estimated supernova rates (reviewed in Manchester & Taylor 1977). Optical and X-ray observations of binary X-ray sources provide the possibility of determining the masses of the neutron stars in these objects

On the emission of neutrinos and gravitational waves in the formation of neutron stars

The energetics involved in the formation of neutron stars in close binaries as a result of supernova explosions are considered. The gravitational binding energy of the neutron star must find proper outlets. The mass ejection and cosmic ray particles can carry away only a small fraction (up to a few per cent) of this energy. Most of the binding energy goes into rotational kinetic energy, gravitational radiation and neutrino emissions. A scenario is considered in which most of the gravitational binding energy goes into rotational kinetic energy and is, ultimately, radiated away as gravitational waves.

Spinning neutron stars and cosmic rays

Spindown of magnetic neutron stars in close binaries by the propeller mechanism, and spinup by accretion are revised. Spindown is more efficient than sometimes claimed. Cosmic rays are probably generated by the propeller mechanism rather than in (single) pulsar winds.

Pulsars and Close Binary Systems

It is first considered what must happen if pulsars (i.e. neutron stars) are formed in close binary systems (CBS), and whether the resulting orbital motion and mass transfer should be observable. As this set of alternatives seems unlikely, there follow suggestions of how one might prevent the formation of neutron stars in close binaries. Finally, it is shown that ‘runaway’ pulsars with velocities larger than about 15 km/sec cannot be produced by isotropic supernova explosions within close binaries, and an alternative explanation is suggested for the observed correlation of periods of pulsars with their distances from the galactic plane.