Plasma disks around compact objects

Considering the azimuthal velocity fields for different radial dependence we obtain the pressure profiles for the thin disk using the general formalism obtained earlier and further look at the profiles of the luminosity flux function using the approach as given recently by Hanawa (1988). It appears that the profile of this function is not very sensitive to change in ther-dependence of the velocity fields.     

Accretion disc dynamics in extreme mass ratio compact binaries

An analysis is presented of a numerical investigation of the dynamics and geometry of accretion discs in binary systems with mass ratios   q = M ₂/ M ₁ < 0.1  , applicable to ultracompact X-ray binaries, AM CVn stars and very short period cataclysmic variables. The steady-state geometry of the disc in the binary reference frame is found to be quite different from that expected at higher mass ratios. For   q ∼ 0.1  , the disc takes on the usual elliptical shape, with the major axis aligned perpendicular to the line of centres of the two stars. However, at smaller mass ratios the elliptical gaseous orbits in the outer regions of the disc are rotated in the binary plane. The angle of rotation increases with gas temperature, but is found to vary inversely with q . At   q = 0.01  , the major axis of these orbits is aligned almost parallel to the line of centres of the two stars. These effects may be responsible for the similar disc structure inferred from Doppler tomography of the AM CVn star GP Com, which has   q = 0.02  . The steady-state geometry at low mass ratios is not predicted by an inviscid, restricted three-body model of gaseous orbits; it is related to the effects of tidal-viscous truncation of the disc near the Roche lobe boundary. Since the disc geometry can be inferred observationally for some systems, it is proposed that this may offer a useful diagnostic for the determination of mass ratios in ultracompact binaries.     

Pulsational instability of accretion disks around compact objects

Linear analysis shows that radial oscillations in accretion disks around compact object are overstable to axisymmetric perturbation under a variety of conditions. Furthermore, numerical simulations confirm that the radial oscillations induce quasi-periodic modulations of the disk luminosity. The disk oscillation model may be responsible for quasi-periodic oscillations (QPOs) observed in low mass X-ray binaries (LMXBs), cataclysmic variables (CVs), and other compact objects.     

Oscillations of fluid disks rotating around compact objects

Frequency of global axisymmetric oscillations of a perfect fluid disk rotating around a compact object is investigated by trial function method. A formula for the frequency of local radial oscillation is presented.     

Plasma disks around compact objects with self-consistent electromagnetic fields

We present a general formalism for discussing the structure and dynamics of plasma disks, around slowly rotating compact objects, having finite viscosity and conductivity. As a test case we consider the situation for the case when the central object is non-rotating and the plasma is with infinite conductivity and show the existence of equilibrium solution for a self-consistent velocity distribution.     

Shear-layer instabilities in accretion discs around magnetized compact objects

In a slab jet model the influence of strong magnetic fields and density contrasts on the development of instabilities caused by velocity contrasts is studied and applied to disc accretion onto magnetized compact object.The perturbations propagating transverse to a magnetic field in external regions are shown not to be stabilized. A strong density contrast at the jet boundary (R=_ex/_in 1) does not stabilize the instability of acoustic resonance type (ARTI), the fundamental symmetric and antisymmetric modes being still unstable for any finite value ofR. At the same time a criticalR-value exists (R1/M ²,M is the Mach number) at which the higher reflection harmonics are stabilized.A comparative analysis is made of ARTI and Kelvin-Helmholtz instability that is developed by surface modes of the interfaces between the disc material and magnetic field (magnetosphere) is performed. ARTI may be responsible for the accreting material penetration into the magnetosphere as well as other mechanisms.We have to note that the difference in names is rather traditional here and is to emphasize the difference in the models.     

Equilibrium structure for a plasma magnetosphere around compact objects

Starting from a set of general equations governing the dynamics of a magneto-fluid around a compact object on curved space time, a fairly simple analytical solution for a test disc having only azimuthal component of velocity has been obtained. The electromagnetic field associated has a modified dipole configuration which admits a reasonable pressure profile for the case of fully relativistic treatment of Keplerian type of velocity distribution     

General relativistic treatment of magnetofluid disk around compact objects

Solutions of magnetofluid accretion disk with negligible self-gravitation have been obtained in the background of the Schwarzschild spacetime for differential as well as rigid rotation. The constraint equations for the magnetic field components have also been derived. It is shown that the magnetic field plays an important role in confining the fluid flow.     

Impact of small bodies on to discs around compact objects — II. Interaction with the disc

We consider a system consisting of a neutron star surrounded by a disc of dense degenerate matter, and study the sequence of events following the impact of comets on to the disc. The direct signature of the impact event is a short burst of high-energy radiation (X-rays to UV, depending on the impact location) emitted from the bubble of hot gas created at the impact site. We assume that the bubble is confined by the magnetic field of the central neutron star. Part of the bubble matter may be channelled along magnetic field lines and rain down on the polar caps. The surface density at the neutron star surface may be sufficient to initiate a runaway thermonuclear reaction. These X-rays or the direct effect of the transferred plasma crossing charge-depleted regions in the outer magnetosphere may re-ignite an otherwise dead pulsar.     

Accretion of matter by condensed objects

The equations of motion for steady-state spherical symmetric flow of matter into or out of a condensed object (e.g. neutron stars, black holes, etc.) are displayed and solved for simple polytropic gases. It appears that infalling matter may be heated as hot as 10¹²K and that X-ray luminosities of the order of 10³⁷ erg s^–1 could result. The two fluid (electrons and ions separately) approach is also examined and it is shown that electrostatic fields of the order of 10⁵ V m^–1 are required near the surface of the object. Such fields are not strong enough to significantly modify the space-time metric.     

Accretion of charged matter by collapsing objects

Re-analysis of the X-ray emission from RS CVn-type binaries has revealed that-contrary to the claims by many authors-their coronal activity is not independent of bolometric luminosity (and thus mass). It becomes clear that activity in late-type stars is also powered-just as in the case of early-type stars-mostly by photospheric radiation.     

Accretion disks in luminous young stellar objects

An observational review is provided of the properties of accretion disks around young stars. It concerns the primordial disks of intermediate- and high-mass young stellar objects in embedded and optically revealed phases. The properties were derived from spatially resolved observations and, therefore, predominantly obtained with interferometric means, either in the radio/(sub)millimeter or in the optical/infrared wavelength regions. We make summaries and comparisons of the physical properties, kinematics, and dynamics of these circumstellar structures and delineate trends where possible. Amongst others, we report on a quadratic trend of mass accretion rates with mass from T Tauri stars to the highest mass young stellar objects and on the systematic difference in mass infall and accretion rates.     

Accretion disc coronae as magnetic reservoirs

Most astrophysical sources powered by accretion on to a black hole, either of stellar mass or supermassive, when observed with hard X-rays show signs of a hot Comptonizing component in the flow, the so-called corona , with observed temperatures and optical depths lying in a narrow range (0.1≲ τ ≲1 and 1×10⁹ K≲ T ≲3×10⁹ K). Here we argue that these facts constitute strong supporting evidence for a magnetically dominated corona. We show that the inferred thermal energy content of the corona, in all black hole systems, is far too low to explain their observed hard X-ray luminosities, unless either the size of the corona is at least of the order of 10³ Schwarzschild radii, or the corona itself is in fact a reservoir , where the energy is mainly stored in the form of a magnetic field generated by a sheared rotator (probably the accretion disc). We briefly outline the main reasons why the former possibility is to be discarded, and the latter preferred.     

General relativistic analysis of structure and stability of charged fluid disks around compact objects

In this paper we give a detailed general relativistic formulation of the study of structure and stability of charged fluid disks around compact objects like black holes neglecting the self-gravitation of the disk itself. Having presented the general equations for equilibrium as well as for perturbations we solve explicitly the cases of rigidly and differentially rotating thin disks, with constant charge density and zero pressure, confined to the equatorial plane of the black hole. By using normal mode analysis we have analysed the stability of such disks under purely radial perturbations and find that the disks are generally stable. On leave of absence from Government College, Jagadalpur 494005     

Accretion effects on compact members of binary stars

We explore the change in the period of axial rotation and in the radius of a magnetized compact star in a binary system, induced by the accretion on it of mass with angular momentum from the surface of its non-compact companion. No specific assumption is made concerning the accretion model, and the primary’s interior is described by the Fermi-Dirac statistics for degenerate matter. The rate of change with time of the period and radius is expressed in terms of the compact primary’s physical parameters and total absolute luminosity. The conditions are fully derived under which the above changes can be positive, negative or even vanish. In the case of the millisecond pulsars in binary X-ray sources the predicted values of the period time derivative, depending on the values of the accretion rate and. the absolute luminosity, can be positive or negative—if not vanishing—and they fall absolutely in the range 10⁻²¹ −10⁻¹⁷ ss⁻¹, in good agreement with current observational data. The corresponding rate of change of radius, either positive or negative, fall in the range of 10⁻³ −10⁻¹ cm y⁻¹. Finally, it is proved that the well-known bursters can be explained by thermonuclear flash due to gravitational instability in the accreted matter, but their explanation as a result of direct contraction could be possible only for quite high accretion rates (>10⁻⁷ M ⊙ y⁻¹). This last result indicates that, in contrast to the accretion-induced change in period, which can be of either sign irrespective of the primary’s age, the accretion-induced non-catastrophic contraction is impossible, while according to repent results the contraction in general is possible for young compact objects.     

Spectral shifts near compact objects

We show that radiation emitted from material freely falling toward a black hole or neutron star cannot be blue-shifted as recently claimed by Cohen and Struble. The relativistic corrections to the classical apparent limb angle are given explicitly for spherical sources in collapse.