1,981 research outputs found
Linear Two-Dimensional MHD of Accretion Disks: Crystalline structure and Nernst coefficient
We analyse the two-dimensional MHD configurations characterising the steady
state of the accretion disk on a highly magnetised neutron star. The model we
describe has a local character and represents the extension of the crystalline
structure outlined in Coppi (2005), dealing with a local model too, when a
specific accretion rate is taken into account. We limit our attention to the
linearised MHD formulation of the electromagnetic back-reaction characterising
the equilibrium, by fixing the structure of the radial, vertical and azimuthal
profiles. Since we deal with toroidal currents only, the consistency of the
model is ensured by the presence of a small collisional effect,
phenomenologically described by a non-zero constant Nernst coefficient (thermal
power of the plasma). Such an effect provides a proper balance of the electron
force equation via non zero temperature gradients, related directly to the
radial and vertical velocity components.
We show that the obtained profile has the typical oscillating feature of the
crystalline structure, reconciled with the presence of viscosity, associated to
the differential rotation of the disk, and with a net accretion rate. In fact,
we provide a direct relation between the electromagnetic reaction of the disk
and the (no longer zero) increasing of its mass per unit time. The radial
accretion component of the velocity results to be few orders of magnitude below
the equatorial sound velocity. Its oscillating-like character does not allow a
real matter in-fall to the central object (an effect to be searched into
non-linear MHD corrections), but it accounts for the out-coming of steady
fluxes, favourable to the ring-like morphology of the disk.Comment: 15 pages, 1 figure, accepted for publication on Modern Physics
Letters
Competitive accretion in embedded stellar cluster
We investigate the physics of gas accretion in young stellar clusters.
Accretion in clusters is a dynamic phenomenon as both the stars and the gas
respond to the same gravitational potential. Accretion rates are highly
non-uniform with stars nearer the centre of the cluster, where gas densities
are higher, accreting more than others. This competitive accretion naturally
results in both initial mass segregation and a spectrum of stellar masses.
Accretion in gas-dominated clusters is well modelled using a tidal-lobe radius
instead of the commonly used Bondi-Hoyle accretion radius. This works as both
the stellar and gas velocities are under the influence of the same
gravitational potential and are thus comparable. The low relative velocity that
results means that the tidal radius is smaller than the Bondi-Hoyle radius in
these systems. In contrast, when the stars dominate the potential and are
virialised, the Bondi-Hoyle radius is smaller than the tidal radius and thus
Bondi-Hoyle accretion is a better fit to the accretion rates.Comment: 11 pages, 11 figures, MNRAS in pres
The evolution of a warped disc around a Kerr black hole
We consider the evolution of a warped disc around a Kerr black hole, under
conditions such that the warp propagates in a wavelike manner. This occurs when
the dimensionless effective viscosity, alpha, that damps the warp is less than
the characteristic angular semi-thickness, H/R, of the disc. We adopt
linearized equations that are valid for warps of sufficiently small amplitude
in a Newtonian disc, but also account for the apsidal and nodal precession that
occur in the Kerr metric. Through analytical and time-dependent studies, we
confirm the results of Demianski & Ivanov, and of Ivanov & Illarionov, that
such a disc takes on a characteristic warped shape. The inner part of the disc
is not necessarily aligned with the equator of the hole, even in the presence
of dissipation. We draw attention to the fact that this might have important
implications for the directionality of jets emanating from discs around
rotating black holes.Comment: 8 pages, 6 figures, to be published in MNRA
Variability Profiles of Millisecond X-Ray Pulsars: Results of Pseudo-Newtonian 3D MHD Simulations
We model the variability profiles of millisecond period X-ray pulsars. We
performed three-dimensional magnetohydrodynamic simulations of disk accretion
to millisecond period neutron stars with a misaligned magnetic dipole moment,
using the pseudo-Newtonian Paczynski-Wiita potential to model general
relativistic effects. We found that the shapes of the resulting funnel streams
of accreting matter and the hot spots on the surface of the star are quite
similar to those for more slowly rotating stars obtained from earlier
simulations using the Newtonian potential. The funnel streams and hot spots
rotate approximately with the same angular velocity as the star. The spots are
bow-shaped (bar-shaped) for small (large) misalignment angles. We found that
the matter falling on the star has a higher Mach number when we use the
Paczynski-Wiita potential than in the Newtonian case.
Having obtained the surface distribution of the emitted flux, we calculated
the variability curves of the star, taking into account general relativistic,
Doppler and light-travel-time effects. We found that general relativistic
effects decrease the pulse fraction (flatten the light curve), while Doppler
and light-travel-time effects increase it and distort the light curve. We also
found that the light curves from our hot spots are reproduced reasonably well
by spots with a gaussian flux distribution centered at the magnetic poles. We
also calculated the observed image of the star in a few cases, and saw that for
certain orientations, both the antipodal hot spots are simultaneously visible,
as noted by earlier authors.Comment: 9 pages, 10 figures, accepted for publication in ApJ; corrected some
typo
The alignment of disk and black hole spins in active galactic nuclei
The inner parts of an accretion disk around a spinning black hole are forced
to align with the spin of the hole by the Bardeen-Petterson effect. Assuming
that any jet produced by such a system is aligned with the angular momentum of
either the hole or the inner disk, this can, in principle provide a mechanism
for producing steady jets in AGN whose direction is independent of the angular
momentum of the accreted material. However, the torque which aligns the inner
disk with the hole, also, by Newton's third law, tends to align the spin of the
hole with the outer accretion disk. In this letter, we calculate this alignment
timescale for a black hole powering an AGN, and show that it is relatively
short. This timescale is typically much less than the derived ages for jets in
radio loud AGN, and implies that the jet directions are not in general
controlled by the spin of the black hole. We speculate that the jet directions
are most likely controlled either by the angular momentum of the accreted
material or by the gravitational potential of the host galaxy.Comment: 4 pages, LateX file, accepted for publication in ApJ Letter
Magnetohydrodynamic turbulence in warped accretion discs
Warped, precessing accretion discs appear in a range of astrophysical
systems, for instance the X-ray binary Her X-1 and in the active nucleus of
NGC4258. In a warped accretion disc there are horizontal pressure gradients
that drive an epicyclic motion. We have studied the interaction of this
epicyclic motion with the magnetohydrodynamic turbulence in numerical
simulations. We find that the turbulent stress acting on the epicyclic motion
is comparable in size to the stress that drives the accretion, however an
important ingredient in the damping of the epicyclic motion is its parametric
decay into inertial waves.Comment: to appear in the proceedings of the 20th Texas Symposium on
Relativistic Astrophysics, J. C. Wheeler & H. Martel (eds.
On the Relative Surface Density Change of Thermally Unstable Accretion Disks
The relations among the relative changes of surface density, temperature,
disk height and vertical integrated pressure in three kinds of thermally
unstable accretion disks were quantitatively investigated by assuming local
perturbations. The surface density change was found to be very small in the
long perturbation wavelength case but can not be ignored in the short
wavelength case. It becomes significant in an optically thin, radiative cooling
dominated disk when the perturbation wavelength is shorter than 15H (H is the
scale height of disk) and in a geometrically thin, optically thick and
radiation pressure dominated disk when the perturbation wavelength is shorter
than 50H. In an optically thick, advection-dominated disk, which is thermally
unstable against short wavelength perturbations, the relative surface density
change is much larger. We proved the positive correlation between the changes
of surface density and temperature in an optically thick, advection- dominated
disk which was previously claimed to be the essential point of its thermal
instability. Moreover, we found an anticorrelation between the changes of disk
height and temperature in an optically thick, advection-dominated disk. This is
the natural result of the absence of appreciable vertical integrated pressure
change.Comment: 10 pages AAS LaTex file, 3 figures, accepted for publication in Ap
Two-dimensional radiation-hydrodynamic model for limit-cycle oscillations of luminous accretion disks
We investigate the time evolution of luminous accretion disks around black
holes, conducting the two-dimensional radiation-hydrodynamic simulations. We
adopt the alpha prescription for the viscosity. The radial-azimuthal component
of viscous stress tensor is assumed to be proportional to the total pressure in
the optically thick region, while the gas pressure in the optically thin
regime. The viscosity parameter, alpha, is taken to be 0.1. We find the
limit-cycle variation in luminosity between high and low states. When we set
the mass input rate from the outer disk boundary to be 100 L_E/c^2, the
luminosity suddenly rises from 0.3L_E to 2L_E, where L_E is the Eddington
luminosity. It decays after retaining high value for about 40 s. Our numerical
results can explain the variation amplitude and duration of the recurrent
outbursts observed in microquasar, GRS 1915+105. We show that the
multi-dimensional effects play an important role in the high-luminosity state.
In this state, the outflow is driven by the strong radiation force, and some
part of radiation energy dissipated inside the disk is swallowed by the black
hole due to the photon-trapping effects. This trapped luminosity is comparable
to the disk luminosity. We also calculate two more cases: one with a much
larger accretion rate than the critical value for the instability and the other
with the viscous stress tensor being proportional to the gas pressure only even
when the radiation pressure is dominant. We find no quasi-periodic light
variations in these cases. This confirms that the limit-cycle behavior found in
the simulations is caused by the disk instability.Comment: 6 pages, 4 figures, accepted for publication in ApJ (ApJ 01 April
2006, v640, 2 issue
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