Radius of the neutron star magnetosphere during disk accretion

The dependence of the spin frequency derivative \(\dot \nu \) of accreting neutron stars with a strongmagnetic field (X-ray pulsars) on the mass accretion rate (bolometric luminosity, L_bol) has been investigated for eight transient pulsars in binary systems with Be stars. Using data from the Fermi/GBM and Swift/BAT telescopes, we have shown that for seven of the eight systems the dependence \(\dot \nu \) (L_bol) can be fitted by the model of angular momentum transfer through an accretion disk, which predicts the relation \(\dot \nu \) ～ L^6/7_bol. Hysteresis in the dependence \(\dot \nu \) (L_bol) has been confirmed in the system V 0332+53 and has been detected for the first time in the systems KS 1947+300, GRO J1008-57, and 1A 0535+26. Estimates for the radius of the neutron star magnetosphere in all of the investigated systems have been obtained. We show that this quantity varies from pulsar to pulsar and depends strongly on the analytical model and the estimates for the neutron star and binary system parameters.     

MHD simulations of jets from accretion disks

We present the MHD simulation including accretion flows in disks, acceleration of outflows from disks, and collimation of the outflows self-consistently. Although it was considered that this kind of simulations only shows the transient phenomena of jets, we found that the outflow and accretion flow reached a quasi-steady state by performing a long-term calculation in a large calculation region. Though the final stage is not exactly the steady state, the acceleration and collimation mechanisms of the outflow were the same as those of the steady theory. The scale of the calculation is approaching to the scale that was observed by the VLBI technique, which provides the current highest resolution for YSO jets.     

Numerical simulations of dissipationless disk accretion

Our goal is to study the regime of disk accretion in which almost all of the angular momentum and energy is carried away by the wind outflowing from the disk in numerical experiments. For this type of accretion the kinetic energy flux in the outflowing wind can exceed considerably the bolometric luminosity of the accretion disk, what is observed in the plasma flow from galactic nuclei in a number of cases. In this paper we consider the nonrelativistic case of an outflow from a cold Keplerian disk. All of the conclusions derived previously for such a system in the self-similar approximation are shown to be correct. The numerical results agree well with the analytical predictions. The inclination angle of the magnetic field lines in the disk is less than 60°, which ensures a free wind outflow from the disk, while the energy flux per wind particle is greater than the particle rotation energy in its Keplerian orbit by several orders of magnitude, provided that the ratio r _A/r ? 1, where r _A is the Alfvénic radius and r is the radius of the Keplerian orbit. In this case, the particle kinetic energy reaches half the maximum possible energy in the simulation region. The magnetic field collimates the outflowing wind near the rotation axis and decollimates appreciably the wind outflowing from the outer disk periphery.     

Wavelength dependence of the polarization of radiation from an accretion disk: Testing accretion disk models

We show that a fundamental choice between various models of an accretion disk around a black hole can be made based on the spectral wavelength distribution of the polarization. This conclusion is based on the possibility of comparing the observed spectral distribution of the polarization with its theoretical values obtained in various accretion disk models. The expected power-law wavelength (frequency) dependences of the polarization for various accretion disk models known in the literature are presented in the table.     

General relativistic MHD simulations of black hole accretion disks and jets

Accretion disks orbiting black holes power high-energy systems such as X-ray binaries and Active Galactic Nuclei. Observations are providing increasingly detailed quantitative information about such systems. This data has been interpreted using standard toy-models that rely on simplifying assumptions such as regular flow geometry and a parameterized stress. Global numerical simulations offer a way to investigate the basic physical dynamics of accretion flows without these assumptions and, in principle, lead to a genuinely predictive theory. In recent years we have developed a fully three-dimensional general relativistic magnetohydrodynamic simulation code that evolves time-dependent inflows into Kerr black holes. Although the resulting global simulations of black hole accretion are still somewhat simplified, they have brought to light a number of interesting results. These include the formation of electro-magnetically dominated jets powered by the black hole’s rotation, and the presence of strong stresses in the plunging region of the accretion flow. The observational consequences of these features are gradually being examined. Increasing computer power and increasingly sophisticated algorithms promise a bright future for the computational approach to black hole accretion.     

Mass accretion by the nuclei of disk galaxies

In this work a model has been proposed to explain how the nucleus of a Galaxy can accumulate mass and becomes supermassive — ultimately giving way to gravitational instability leading to an explosion in the nucleus. The process may be repeated many times during the life-span of a Galaxy. The mass shed by the evolved stars populating the central region of the Galaxy can be attracted toward the nuclear core by gravitational pull. Since the incident gas possesses rotational velocity, the centrifugal repulsion of the gas may balance the gravitational pull of the nucleus; thus infall of mass into the nucleus will ordinarily be inhibited. But dissipative agents — such as the prevailing magnetic field and the viscosity of gas — may be sufficient to destroy the rotational velocity of the incident gas and keep the accretion process efficient. The correlation between rotational velocity of gas and its distance from the centre of the Galaxy has been deduced. The radial equation of motion has been solved and the time-scale during which the nucleus accumulates mass sufficient for explosion, has been estimated.     

Formation of astrophysical jets by a contracting magnetic accretion disk

In the present paper, we discuss an MHD model for the formation of astrophysical jets, in which the directed flows are ejected along the rotation axis of an accretion disk formed from a cloud having a large scale magnetic field parallel to the angular momentum axis of the disk. The acceleration of jets is due to thej×B force in the relaxing magnetic twist which is produced by the rotation of the disk. The characteristic features of the jets, predicted by our mechanism and hopefully to be proven by observations, are the helical velocity and the hollow cylindrical shape of the jet, with a diameter of roughly the size of the region from which the acceretion disk collected its mass. Justification for the assumption of the perpendicular orientation of the disk, or the parallelism of the jets, to the external magnetic field may be provided by the fact that the component of rotation whose axis is perpendicular to the field may have been damped in the earlier phase of the cloud contraction.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 Septemper–6 October, 1984.     

Formation of the prelunar accretion disk

According to the single-impact hypothesis for forming the Moon, the angular momentum needed for the present Earth-Moon system can be imparted to the proto-Earth by a collision with a body having one-tenth of the mass or more. The collision must vaporize a large amount of rock which must stay in the form of vapor after expanding in density by a factor of several, so that pressure gradients can accelerate significant amounts of the matter into orbital motion about the proto-Earth. A successful theory must put considerably more than a lunar mass into orbit, having considerably more angular momentum than is needed to assemble a lunar mass in orbit at 3 Earth radii. Such a collision has been simulated by a particular form of a particle-in-cell representation of hydrodynamics and 78 cases have been run representing variations in a variety of parameters. A significant fraction of the cases were successful in creating a satisfactory prelunar accretion disk. A fairly common characteristic of these cases was the presence of an excess velocity in the collision (above that of a parabolic orbit), implying that the projectile involved in the collision existed in an Earth-crossing orbit of significant ellipticity. A majority of the mass of the prelunar accretion disk is contributed by the projectile.     

On the torque exerted by a warped,magnetically threaded accretion disk

Most astrophysical accretion disks are likely to be warped.In X-ray binaries,the spin evolution of an accreting neutron star is critically dependent on the interaction between the neutron star magnetic field and the accretion disk.There have been extensive investigations on the accretion torque exerted by a coplanar disk that is magnetically threaded by the magnetic field lines from the neutron stars,but relevant works on warped/tilted accretion disks are still lacking.In this paper we develop a simplified twocomponent model,in which the disk is comprised of an inner coplanar part and an outer,tilted part.Based on standard assumption on the formation and evolution of the toroidal magnetic field component,we derive the dimensionless torque and show that a warped/titled disk is more likely to spin up the neutron star compared with a coplanar disk.We also discuss the possible influence of various initial parameters on the torque.     

Mass accretion by the nuclei of disk galaxies,II

Repeated explosions in the nuclei of galaxies are now accepted as observationally established phenomena. Each explosion leads to the ejection of gas from the central region of a galaxy with velocities depending on the strength of the explosive event. In the process the nucleus temporarily becomes gas-deficient. It is suggested that the mass los is replenished by the accretion of the mass which is shed by those evolved stars in the galactic bulge that possess relatively low rotational velocities. The gas to be accreted is assumed to be magnetized. In the present model, the accretion rate has been assumed to be a function of both radial distance and time. The cross-radial equation of motion has been solved to derive the expression for the rotational velocity which is found to bealmost linear with the radial distance from the centre. The radial equation has been solved to calculate the time-scale over which the nucleus accumulates sufficient mass to undergo instability and suffer explosion. The calculated time-scale range from few multiples of 10⁷ to a few multiples of 10⁸ yr. This range agrees very well with that as has been suggested on the basis of observation in the case of our own Galaxy.     

Physics of the primitive solar accretion disk

The theory of viscous accretion disks developed by Lynden-Bell and Pringle has been applied to the evolution of the primitive solar nebula. The additional physical input needed to determine the structure of the disk is described. A series of calculations was carried out using a steady flow approximation to explore the effects on the disk properties of variations in such parameters as the angular momentum and accretion rate of the infalling material from a collapsing interstellar cloud fragment. The more detailed evolutionary calculations involved five cases with various combinations of parameters. It was concluded that the late stages of evolution of the disks would be dominated by the effects of mass loss from the expansion of a hot disk corona into space, and the effects of this were included in the evolutionary calculations. A new theory of comet formation is formulated upon these results. The most important result is the conclusion, which appears to be inescapable, that the primitive solar accretion disk was repeatedly unstable against axisymmetric perturbations, in which rings would form and collapse upon themselves, with the subsequent formation of giant gaseous protoplanets.     

On the computation of accretion disk spectra

Spectra of accretion disks in dwarf novae and some novalike stars have been computed. Many simplifications to the general model of the nature of a cataclysmic variable are needed to make the system numerically tractable. The necessity, justification, and implications of such simplifications are discussed together with the influences of some system parameters on the disk radiation.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.     

Evidence for initial jet formation by an accretion disk in the radio galaxy M87

The nearby E0 galaxy M87 in the Virgo Cluster contains one of the nearest examples of a powerful non-thermal jet source. In addition, HST spectroscopy of its nucleus give strong evidence for the presence of a ≈3×10⁹ M_⊙ central black hole (corresponding to Schwarzschild radius R_s∼0.0004 pc). These two facts together make M87 the best possible target for studying the initial jet formation and collimation process. Herein we report new 43 GHz VLBI observations of the nucleus of this galaxy, which clearly show the jet opening angle expanding rapidly as one approaches the core on scales ∼0.01 pc. We believe we have, for the first time, imaged the initial collimation region of a powerful extragalactic jet. We find significant collimation occurring on scales of 30–100 R_s, and argue this is consistent with expectations for poloidal collimation by a rotating accretion disk.     

Simulations of jet formation from a magnetized accretion disk

Axisymmetric magnetohydrodynamic (MHD) simulations have been made of the formation of jets from a Keplerian disk threaded by a magnetic field. The disk is treated as a boundary condition, where matter with high specific entropy is ejected with a Keplerian azimuthal speed and a poloidal speed less than the slow magnetosonic velocity, and where boundary conditions on the magnetic fields correspond to a highly conducting disk. Initially, the space above the disk, the corona, is filled with high specific entropy plasma in the thermal equilibrium in the gravitational field of the central object. The initial magnetic field is poloidal and is represented by the superposition of the fields of monopoles located below the plane of the disk.The rotation of the disk twists the initial poloidal magnetic field lines, and this twist propagates into the corona pushing matter into jet-like outflow in a cylindrical region. After the first switch-on wave, which originates during the first rotation period of the inner radius of the disk, the matter outflowing from the disk starts to flow and accelerate in thez-direction owing to both the magnetic and pressure gradient forces. The flow accelerates through the slow magnetosonic and Alfvén surfaces and at larger distances through the fast magnetosonic surface. The flow velocity of the jet is approximately parallel to thez-axis, with the collimation mainly a result of the pinching force of the toroidal magnetic field. The energy flux of the flow increases with increasing magnetic field strength on the disk. Some of the cases studied have been run for long times, 60 rotation periods of the inner radius of the disk, and show indications of approaching a stationary state.     

Structure of the satellitary accretion disk of Saturn

A detailed computation on the equilibrium structure of an accretion disk around Saturn from which the regular satellites presumably originated is reported. Such a disk is the predecessor of the self-dissipating disk that is formed when the mass infall stops (Cassen and Moosman, 1981, Icarus48, 353–376). When determining the disk structure local energy balance was assumed. Convention was taken into account by introducing local energy dissipation and, in an approximate manner, sonic convection. Changes in the disk structure were investigated by varying the free parameters, i.e., the external flux from both the protosun and the protoplanet, the abundance of dust and the strength of turbulence. It has been verified that the external energy flux does not play an important role in the evolution of the disk structure. Models characterized by either longer times (?3 10³ year) or a noticeable depletion of condensable elements (10^?2 times less than the solar value) have a total mass of the order of 0.34?0.1 times the mass of the regular satellites increased by the mass of the light elements. Low turbulence models (Reynolds critical number $\text{Re}{}^1\text{= 150}$) are characterized approximately by a total mass twice as large the mass of the regular satellites. All the studied models present a temperature distribution that allows the condensation of iron, silicate, and, in the outer regions, ice grains. All models but the one with 10^?2 of the solar value of condensable elements are characterized by a wide convective region that contains the formation zone of the regular satellites.     

Stellar dynamics in a galactic centre surrounded by a massive accretion disc — I. Newtonian description

The long-term evolution of stellar orbits bound to a massive centre is studied in order to understand the cores of star clusters in central regions of galaxies. Stellar trajectories undergo tiny perturbations, the origins of which are twofold: (i) the gravitational field of a thin gaseous disc surrounding the galactic centre, and (ii) cumulative drag arising from successive interactions of the stars with the material of the disc. Both effects are closely related because they depend on the total mass of the disc, assumed to be a small fraction of the central mass. It is shown that, in contrast to previous works, most of the retrograde (with respect to the disc) orbits are captured by the central object, presumably a massive black hole. Initially prograde orbits are also affected, so that statistical properties of the central star cluster in quasi-equilibrium may differ significantly from those deduced in previous analyses.     

Temperature profile of accretion disk around a kerr black hole

We use a rigorous, general relativistic, viscous, fluiddynamical equation to calculate the temperature profile of the accretion disk around a Kerr black hole. Our result shows that there is indeed a maximum in the profile. If the radiation in the disk is black-body radiation, then the result is independent of the viscosity mechanism. Using a free-free radiation and the Lα viscosity law, our results are similar to our previous findings [1]. Our calculation also shows that within the radius of the temperature maximum, there exists a rapidly cooling ring-shaped region, possessing characteristics that are not found in the case with the usual temperature profile.     

Unipolar induction of a magnetized accretion disk around a black hole

The structure and magnitude of the electromagnetic field produced by a rotating accretion disk around a black hole were determined. The disk matter is assumed to be a magnetized plasma with a frozenin poloidal magnetic field. The vacuum approximation is used outside the disk.