Spin evolution of neutron stars in accretion powered pulsars
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This is a review of our understanding of the evolution of the spin periods of neutron stars in accretion‐powered binary x‐ray pulsars during the course of evolution of these binary systems. I discuss how neutron stars born in binaries as rotation‐powered pulsars are initially spun down by electromagnetic and plasma torques, until accretion of matter from the companion star begins. I describe how accretion torques spin up and spin down the star during the main x‐ray emission phase of neutron stars accreting from disks, winds, or equatorial rings produced by mass loss from their companions. I outline the final phase of spinup of the neutron stars into recycled rotation‐powered pulsars.Keywords:
X-ray pulsar
X-ray binary
X-ray burster
Compact star
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Cygnus X-3 is a strong X-ray source ( erg s-1) which is thought to consist of a compact object accreting matter from a helium star. We analytically find that the estimated ranges of mass-loss rate and orbital-period derivative for Cyg X-3 are consistent with two models: i) the system is detached and the mass loss from the system comes from the stellar wind of a massive helium star, of which only a fraction that allows for the observed X-ray luminosity is accreted, or ii) the system is semidetached and a Roche-lobe-overflowing low- or moderate-mass helium donor transfers mass to the compact object, followed by ejection of its excess over the Eddington rate from the system. These analytical results appear to be consistent with evolutionary calculations. By means of population synthesis we find that currently in the Galaxy there may exist ~1 X-ray binary with a black hole that accretes from a Wolf-Rayet star and ~1 X-ray binary in which a neutron star accretes matter from a Roche-lobe-overflowing helium star with mass . Cyg X-3 is probably one of these systems.
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X-ray binary
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Eddington luminosity
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Cyg X-3 is a highly interesting accreting X-ray binary, emitting from the radio to high-energy gamma-rays. It consists of a compact object wind-fed by a Wolf-Rayet (WR) star, but the masses of the components and the mass-loss rate have been a subject of controversies. Here, we determine its masses, inclination, and the mass-loss rate using our derived relationship between the mass-loss rate and the mass for WR stars of the WN type, published infrared and X-ray data, and a relation between the mass-loss rate and the binary period derivative (observed to be >0 in Cyg X-3). Our obtained mass-loss rate is almost identical to that from two independent estimates and consistent with other ones, which strongly supports the validity of this solution. The found WR and compact object masses are 10.3_{-2.8}^{+3.9}, 2.4_{-1.1}^{+2.1} solar masses, respectively. Thus, our solution still allows for the presence of either a neutron star or a black hole, but the latter only with a low mass. However, the radio, infrared and X-ray properties of the system suggest that the compact object is a black hole. Such a low-mass black-hole could be formed via accretion-induced collapse or directly from a supernova.
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X-ray binaries are binary stellar systems containing a compact object and a normal companion star which are gravitationally bound and rotate about a common center of mass. The compact object accretes matter from the companion star. The accreted matter may have a high angular momentum and hence follow a Keplarian orbit about the compact object. It slowly spirals inward as its angular momentum is redistributed via viscous forces and forms an accreting disk before being finally accreted onto the compact object. The compact object that is accreting matter may either be a neutron star or a black hole. X-ray binaries can be broadly classified into two classes depending on the mass of the companion star. Low Mass X-ray Binaries (LMXBs) have companion star masses and accrete mass via Roche lobe overflow of the companion star. High Mass X-ray Binaries (HMXBs) have companion star masses and in these systems the compact object accretes matter from the high velocity stellar winds of the companion star.
For the work and results that are presented in the thesis we have studied the orbital evolution, apsidal motion and long term flux variations in High mass X-ray binaries which have a neutron star compact object with very high magnetic field of the order of B ~ 1012 G. Due to the high magnetic field, the accretion disk is disrupted at the Alfven radius where the magnetic field pressure equals the ram pressure of the infalling matter. From that boundary, the flow of the infalling matter will be guided by the magnetic field lines. The infalling matter will follow these lines, finally falling onto the magnetic poles with velocity nearly equal to the free fall velocity and form an accretion column over the magnetic poles. A hot spot is formed at both the magnetic poles and high energy photons are emitted from these regions. Inverse Compton scattering of these photons by high energy electrons in the accretion column can produce hard X-rays. If the optical depth of the accretion column is low, the radiation comes along the magnetic axis forming a pencil beam whereas if the optical depth is high, radiation escapes tangential to the accretion column forming a fan beam. Since the neutron star is rotating about its rotation axis, the radiation beam directed along magnetic axis non-aligned with the rotation axis will sweep across the sky. Whenever this beam of rotating radiation is aligned with the line of sight, a pulse of X-ray radiation is detected. Hence these systems are also called X-ray Binary Pulsars (XBP). These pulses are emitted at equal intervals of time, where the time between the emission of two pulses is the spin period of the neutron star. But since the neutron star is in a binary orbit, the arrival time of pulses as recorded by an observer will be delayed or advanced due to the motion of the neutron star. When the neutron star is moving towards the observer, the pulses arrive faster and when the neutron star is moving away from the observer, the pulses are delayed. These delays or advances of the arrival time of pulses…
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X-ray burster
X-ray pulsar
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Abstract I discuss the nature of the compact X-ray source inside the supernova remnant RCW 103. Several models, based on the accretion of matter onto a compact object such as a neutron star or a black hole (isolated or binary), are analysed. I show that it is more likely that the X-ray source is an accreting neutron star than an accreting black hole. I also argue that models of a binary system with an old accreting neutron star are most favoured.
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