35 research outputs found
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.
The response of a turbulent accretion disc to an imposed epicyclic shearing motion
We excite an epicyclic motion, whose amplitude depends on the vertical
position, , in a simulation of a turbulent accretion disc. An epicyclic
motion of this kind may be caused by a warping of the disc. By studying how the
epicyclic motion decays we can obtain information about the interaction between
the warp and the disc turbulence. A high amplitude epicyclic motion decays
first by exciting inertial waves through a parametric instability, but its
subsequent exponential damping may be reproduced by a turbulent viscosity. We
estimate the effective viscosity parameter, , pertaining to
such a vertical shear. We also gain new information on the properties of the
disc turbulence in general, and measure the usual viscosity parameter,
, pertaining to a horizontal (Keplerian) shear. We find that,
as is often assumed in theoretical studies, is approximately
equal to and both are much less than unity, for the field
strengths achieved in our local box calculations of turbulence. In view of the
smallness () of and we conclude
that for the timescale for diffusion
or damping of a warp is much shorter than the usual viscous timescale. Finally,
we review the astrophysical implications.Comment: 12 pages, 18 figures, MNRAS accepte
The effects of vertical outflows on disk dynamos
We consider the effect of vertical outflows on the mean-field dynamo in a
thin disk. These outflows could be due to winds or magnetic buoyancy. We
analyse both two-dimensional finite-difference numerical solutions of the
axisymmetric dynamo equations and a free-decay mode expansion using the
thin-disk approximation. Contrary to expectations, a vertical velocity can
enhance dynamo action, provided it is not too strong. In the nonlinear regime
this can lead to super-exponential growth of the magnetic field.Comment: 14 pages, final version after referee comments, accepted in A&
Possible potentials responsible for stable circular relativistic orbits
Bertrand's theorem in classical mechanics of the central force fields
attracts us because of its predictive power. It categorically proves that there
can only be two types of forces which can produce stable, circular orbits. In
the present article an attempt has been made to generalize Bertrand's theorem
to the central force problem of relativistic systems. The stability criterion
for potentials which can produce stable, circular orbits in the relativistic
central force problem has been deduced and a general solution of it is
presented in the article. It is seen that the inverse square law passes the
relativistic test but the kind of force required for simple harmonic motion
does not. Special relativistic effects do not allow stable, circular orbits in
presence of a force which is proportional to the negative of the displacement
of the particle from the potential center.Comment: 11 pages, Latex fil
Hypercritical Advection Dominated Accretion Flow
In this note we study the accretion disc that arises in hypercritical
accretion of onto a neutron star while it is in
common envelope evolution with a massive companion. In order to raise the
temperature high enough that the disc might cool by neutrino emission,
Chevalier found a small value of the -parameter, where the kinematic
coefficient of shear viscosity is , with the velocity
of sound and the disc height; namely, was necessary
for gas pressure to dominate. He also considered results with higher values of
, pointing out that radiation pressure would then predominate. With
these larger 's, the temperatures of the accreting material are much
lower, \lsim 0.35 MeV. The result is that neutrino cooling during the flow is
negligible, satisfying very well the advection dominating conditions. The low
temperature of the accreting material means that it cannot get rid of its
energy rapidly by neutrino emission, so it piles up, pushing its way through
the accretion disc. An accretion shock is formed, far beyond the neutron star,
at a radius \gsim 10^8 cm, much as in the earlier spherically symmetric
calculation, but in rotation. Two-dimensional numerical simulation shows that
an accretion disc is reformed inside of the accretion shock, allowing matter to
accrete onto the neutron star with pressure high enough so that neutrinos can
carry off the energy.Comment: 6 pages, ApJ, submitte
An Incoherent Dynamo in Accretion Disks
We use the mean-field dynamo equations to show that an incoherent alpha
effect in mirror-symmetric turbulence in a shearing flow can generate a large
scale, coherent magnetic field. We illustrate this effect with simulations of a
few simple systems. In accretion disks, this process can lead to axisymmetric
magnetic domains whose radial and vertical dimensions will be comparable to the
disk height. This process may be responsible for observations of dynamo
activity seen in simulations of dynamo-generated turbulence involving, for
example, the Balbus-Hawley instability. In this case the magnetic field
strength will saturate at times the ambient pressure in real
accretion disks. The resultant dimensionless viscosity will be of the same
order. In numerical simulations the azimuthal extent of the simulated annulus
should be substituted for . We compare the predictions of this model to
numerical simulations previously reported by Brandenburg et al. (1995). In a
radiation pressure dominated environment this estimate for viscosity should be
reduced by a factor of due to magnetic buoyancy.Comment: 23 pages, uses aaste
Oscillations of tori in the pseudo-Newtonian potential
Context. The high-frequency quasi-periodic oscillations (HF QPOs) in neutron
star and stellar-mass black hole X-ray binaries may be the result of a
resonance between the radial and vertical epicyclic oscillations in strong
gravity. Aims. In this paper we investigate the resonant coupling between the
epicyclic modes in a torus in a strong gravitational field. Methods. We perform
numerical simulations of axisymmetric constant angular momentum tori in the
pseudo-Newtonian potential. The epicyclic motion is excited by adding a
constant radial velocity to the torus. Results. We verify that slender tori
perform epicyclic motions at the frequencies of free particles, but the
epicyclic frequencies decrease as the tori grow thicker. More importantly, and
in contrast to previous numerical studies, we do not find a coupling between
the radial and vertical epicyclic motions. The appearance of other modes than
the radial epicyclic motion in our simulations is rather due to small numerical
deviations from exact equilibrium in the initial state of our torus.
Conclusions. We find that there is no pressure coupling between the two
axisymmetric epicyclic modes as long as the torus is symmetric with respect to
the equatorial plane. However we also find that there are other modes in the
disc that may be more attractive for explaining the HF QPOs.Comment: 8 pages, 9 figure
Magnetic shear-driven instability and turbulent mixing in magnetized protostellar disks
Observations of protostellar disks indicate the presence of the magnetic
field of thermal (or superthermal) strength. In such a strong magnetic field,
many MHD instabilities responsible for turbulent transport of the angular
momentum are suppressed. We consider the shear-driven instability that can
occur in protostellar disks even if the field is superthermal. This instability
is caused by the combined influence of shear and compressibility in a
magnetized gas and can be an efficient mechanism to generate turbulence in
disks. The typical growth time is of the order of several rotation periods.Comment: 8 pages, 6 figures, A&A to appea
Torque bistability in the interaction between a neutron star magnetosphere and a thin accretion disc
We present a time-dependent model of the interaction between a neutron star
magnetosphere and a thin (Shakura-Sunyaev) accretion disc, where the extent of
the magnetosphere is determined by balancing outward diffusion and inward
advection of the stellar magnetic field at the inner edge of the disc. The
nature of the equilibria available to the system is governed by the magnetic
Prandtl number Pm and the ratio \xi of the corotation radius to the Alfven
radius. For \xi > Pm^0.3, the system can occupy one of two stable states, where
the torques are of opposite signs. If the star is spinning up initially, in the
absence of extraneous perturbations, \xi decreases until the spin-up
equilibrium vanishes, the star subsequently spins down, and the torque
asymptotes to zero. Vortex-in-cell simulations of the Kelvin-Helmholtz
instability suggest that the transport speed across the mixing layer between
the disc and magnetosphere is less than the shear speed when the layer is thin,
unlike in previous models.Comment: 11 pages, 10 figure
An alpha theory of time-dependent warped accretion discs
The non-linear fluid dynamics of a warped accretion disc was investigated in
an earlier paper by developing a theory of fully non-linear bending waves in a
thin, viscous disc. That analysis is here extended to take proper account of
thermal and radiative effects by solving an energy equation that includes
viscous dissipation and radiative transport. The problem is reduced to simple
one-dimensional evolutionary equations for mass and angular momentum, expressed
in physical units and suitable for direct application. This result constitutes
a logical generalization of the alpha theory of Shakura & Sunyaev to the case
of a time-dependent warped accretion disc. The local thermal-viscous stability
of such a disc is also investigated.Comment: 16 pages, 3 figures, to be published in MNRA