541 research outputs found
Stability of the Magnetopause of Disk-Accreting Rotating Stars
We discuss three modes of oscillation of accretion disks around rotating
magnetized neutron stars which may explain the separations of the kilo-Hertz
quasi periodic oscillations (QPO) seen in low mass X-ray binaries. The
existence of these compressible, non-barotropic magnetohydrodynamic (MHD) modes
requires that there be a maximum in the angular velocity of
the accreting material larger than the angular velocity of the star ,
and that the fluid is in approximately circular motion near this maximum rather
than moving rapidly towards the star or out of the disk plane into funnel
flows. Our MHD simulations show this type of flow and profile.
The first mode is a Rossby wave instability (RWI) mode which is radially
trapped in the vicinity of the maximum of a key function at
. The real part of the angular frequency of the mode is
, where is the azimuthal mode number.
The second mode, is a mode driven by the rotating, non-axisymmetric component
of the star's magnetic field. It has an angular frequency equal to the star's
angular rotation rate . This mode is strongly excited near the radius
of the Lindblad resonance which is slightly outside of . The third mode
arises naturally from the interaction of flow perturbation with the rotating
non-axisymmetric component of the star's magnetic field. It has an angular
frequency . We suggest that the first mode with is associated
with the upper QPO frequency, ; that the nonlinear interaction of the
first and second modes gives the lower QPO frequency, ;
and that the nonlinear interaction of the first and third modes gives the lower
QPO frequency , where .Comment: 10 pages, 7 figure
Possible Quasi-Periodic Oscillations from Unstable Accretion: 3D MHD Simulations
We investigate the photometric variability of magnetized stars, particularly
neutron stars, accreting through a magnetic Rayleigh-Taylor-type instability at
the disk-magnetosphere interface, and compare it with the variability during
stable accretion, with the goal of looking for possible quasi-periodic
oscillations. The lightcurves during stable accretion show periodicity at the
star's frequency and sometimes twice that, due to the presence of two funnel
streams that produce antipodal hotspots near the magnetic poles. On the other
hand, lightcurves during unstable accretion through tongues penetrating the
magnetosphere are more chaotic due to the stochastic behaviour of the tongues,
and produce noisier power spectra. However, the power spectra do show some
signs of quasi-periodic variability. Most importantly, the rotation frequency
of the tongues and the resulting hotspots is close to the inner-disk orbital
frequency, except in the most strongly unstable cases. There is therefore a
high probability of observing QPOs at that frequency in longer simulations. In
addition, the lightcurves in the unstable regime show periodicity at the star's
rotation frequency in many of the cases investigated here, again except in the
most strongly unstable cases which lack funnel flows and the resulting
antipodal hotspots. The noisier power spectra result in the fractional rms
amplitudes of the Fourier peaks being smaller.
We also study in detail the effect of the misalignment angle between the
rotation and magnetic axes of the star on the variability, and find that at
misalignment angles , the star's period always appears in the
lightcurves.Comment: 14 pages, 16 figures, accepted by MNRAS. v2 comments: significant
revision. v3 comments: after referee report. Rewrote QPO section (4.5). v4
comments: final versio
Warps, bending and density waves excited by rotating magnetized stars: results of global 3D MHD simulations
We report results of the first global three-dimensional magnetohydrodynamic
simulations of the waves excited in an accretion disc by a rotating star with a
dipole magnetic field misaligned from the star's rotation axis (which is
aligned with the disc axis). The main results are the following: (1) If the
magnetosphere of the star corotates approximately with the inner disc, then we
observe a strong one-armed bending wave (a warp). This warp corotates with the
star and has a maximum amplitude between corotation radius and the radius of
the vertical resonance. The disc's center of mass can deviate from the
equatorial plane up to the distance of z_w\approx 0.1 r. However, the effective
height of the warp can be larger, h_w \approx 0.3 r due to the finite thickness
of the disc. Stars with a range of misalignment angles excite warps. However,
the amplitude of the warps is larger for misalignment angles between 15 and 60
degrees. (2) If the magnetosphere rotates slower, than the inner disc, then a
bending wave is excited at the disc-magnetosphere boundary, but does not form a
large-scale warp. Instead, high-frequency oscillations become strong at the
inner region of the disc. These are (a) trapped density waves which form inside
the radius where the disc angular velocity has a maximum, and (b) inner bending
waves which appear in the case of accretion through magnetic Raleigh-Taylor
instability. These two types of waves are connected with the inner disc and
their frequencies will vary with accretion rate. Bending oscillations at lower
frequencies are also excited including global oscillations of the disc. In
cases where the simulation region is small, slowly-precessing warp forms.
Simulations are applicable to young stars, cataclysmic variables, and accreting
millisecond pulsars.Comment: 26 pages, 25 figure
Relativistic Jets from Accretion Disks
The jets observed to emanate from many compact accreting objects may arise
from the twisting of a magnetic field threading a differentially rotating
accretion disk which acts to magnetically extract angular momentum and energy
from the disk. Two main regimes have been discussed, hydromagnetic jets, which
have a significant mass flux and have energy and angular momentum carried by
both matter and electromagnetic field and, Poynting jets, where the mass flux
is small and energy and angular momentum are carried predominantly by the
electromagnetic field. Here, we describe recent theoretical work on the
formation of relativistic Poynting jets from magnetized accretion disks.
Further, we describe new relativistic, fully-electromagnetic, particle-in-cell
simulations of the formation of jets from accretion disks. Analog Z-pinch
experiments may help to understand the origin of astrophysical jets.Comment: 7 pages, 3 figures, Proc. of High Energy Density Astrophysics Conf.,
200
A diagnosis on torque reversals in 4U 1626-67
Several X-ray pulsars have been observed to experience torque reversals,
which provide important observational clues to the interaction between the
neutron star magnetic field and the accretion disk. We review the current
models proposed for the torque reversals and discuss their viability based on
the observations of the quasi-periodic oscillations (QPOs) in 4U 1626-67. Most
of these models seem to be incompatible with the evolution of the QPO
frequencies if they are interpreted in terms of the beat frequency model. We
suggest that winds or outflows from the neutron star and the accretion disk may
play an important role in accounting for the spin-down in disk-fed neutron
stars.Comment: 7 pages, accepted for publication in A&
Kelvin-Helmholtz Instability of the Magnetopause of Disc-Accreting Stars
This work investigates the short wavelength stability of the magnetopause
between a rapidly-rotating, supersonic, dense accretion disc and a
slowly-rotating low-density magnetosphere of a magnetized star. The
magnetopause is a strong shear layer with rapid changes in the azimuthal
velocity, the density, and the magnetic field over a short radial distance and
thus the Kelvin-Helmholtz (KH) instability may be important. The plasma
dynamics is treated using non-relativistic, compressible (isentropic)
magnetohydrodynamics. It is necessary to include the displacement current in
order that plasma wave velocities remain less than the speed of light. We focus
mainly on the case of a star with an aligned dipole magnetic field so that the
magnetic field is axial in the disc midplane and perpendicular to the disc flow
velocity. However, we also give results for cases where the magnetic field is
at an arbitrary angle to the flow velocity. For the aligned dipole case the
magnetopause is most unstable for KH waves propagating in the azimuthal
direction perpendicular to the magnetic field which tends to stabilize waves
propagating parallel to it. The wave phase velocity is that of the disc matter.
A quasi-linear theory of the saturation of the instability leads to a
wavenumber () power spectrum of the density and temperature
fluctuations of the magnetopause, and it gives the mass accretion and angular
momentum inflow rates across the magnetopause. For self-consistent conditions
this mass accretion rate will be equal to the disc accretion rate at large
distances from the magnetopause.Comment: 8 pages, 7 figure
Launching of Conical Winds and Axial Jets from the Disk-Magnetosphere Boundary: Axisymmetric and 3D Simulations
We investigate the launching of outflows from the disk-magnetosphere boundary
of slowly and rapidly rotating magnetized stars using axisymmetric and
exploratory 3D magnetohydrodynamic (MHD) simulations. We find long-lasting
outflows in both cases. (1) In the case of slowly rotating stars, a new type of
outflow, a conical wind, is found and studied in simulations. The conical winds
appear in cases where the magnetic flux of the star is bunched up by the disk
into an X-type configuration. The winds have the shape of a thin conical shell
with a half-opening angle 30-40 degrees. The conical winds may be responsible
for episodic as well as long-lasting outflows in different types of stars. (2)
In the case of rapidly rotating stars (the "propeller regime"), a two-component
outflow is observed. One component is similar to the conical winds. A
significant fraction of the disk matter may be ejected into the winds. A second
component is a high-velocity, low-density magnetically dominated axial jet
where matter flows along the opened polar field lines of the star. The jet has
a mass flux about 10% that of the conical wind, but its energy flux (dominantly
magnetic) can be larger than the energy flux of the conical wind. The jet's
angular momentum flux (also dominantly magnetic) causes the star to spin-down
rapidly. Propeller-driven outflows may be responsible for the jets in
protostars and for their rapid spin-down. The jet is collimated by the magnetic
force while the conical winds are only weakly collimated in the simulation
region.Comment: 29 pages and 29 figures. This version has a major expansion after
comments by a referee. The 1-st version is correct but mainly describes the
conical wind. This version describes in greater detail both the conical winds
and the propeller regime. Accepted to the MNRA
Magnetically Controlled Accretion Flows onto Young Stellar Objects
(abridged) Accretion from disks onto young stars is thought to follow
magnetic field lines from the inner disk edge to the stellar surface. The
accretion flow thus depends on the geometry of the magnetic field. This paper
extends previous work by constructing a collection of orthogonal coordinate
systems, including the corresponding differential operators, where one
coordinate traces the magnetic field lines. This formalism allows for an
(essentially) analytic description of the geometry and the conditions required
for the flow to pass through sonic points. Using this approach, we revisit the
problem of magnetically controlled accretion flow in a dipole geometry, and
then generalize the treatment to consider magnetic fields with multiple
components, including dipole, octupole, and split monopole contributions. This
approach can be generalized further to consider more complex magnetic field
configurations. Observations indicate that accreting young stars have
substantial dipole and octupole components, and that accretion flow is
transonic. If the effective equation of state for the fluid is too stiff, the
flow cannot pass smoothly through the sonic points in steady state. For a
multipole field of order \ell, we derive a constraint on the polytropic index,
n>\ell+3/2, required for steady transonic flow to reach free-fall velocities.
For octupole fields, inferred on surfaces of T Tauri stars, n>9/2, so that the
flow must be close to isothermal. The inclusion of octupole field components
produces higher densities at the stellar surface and smaller hot spots, which
occur at higher latitudes; the magnetic truncation radius is also modified.
This contribution thus increases our understanding of magnetically controlled
accretion for young stellar objects and can be applied to a variety of
additional astrophysical problems.Comment: 50 pages, 8 figures, accepted to Ap
A Disc-Corona Model for a Rotating Black Hole
We propose a disc-corona model in which a geometrically thin, optically thick
disc surrounds a Kerr black hole, and magnetic fields exert a time-steady
torque on the inner edge of the accretion disc. The analytical expression of
the total gravitational power is derived from the thin-disc dynamics equations
by using this new boundary condition. It is shown that the magnetic torque can
considerably enhance the amount of energy released in the disc-corona system.
Furthermore, the global solutions of this disc-corona system are obtained
numerically. We find that the fraction of the power dissipated into the corona
in the total for such disc-corona system increases with the increasing
dimensionless black hole spin parameter , but is insensitive on the
which is the additional radiative efficiency parameter
relevant to magnetic torque, for . In addition, the
emerged spectra from this disc-corona system are simulated by using Monte-Carlo
method, and the effect of the different parameters on the output spectra is
discussed
Observed luminosity spread in young clusters and Fu Ori stars: a unified picture
The idea that non steady accretion during the embedded phase of protostar
evolution can produce the observed luminosity spread in the Herzsprung-Russell
diagram (HRD) of young clusters has recently been called into question.
Observations of Fu Ori, for instance, suggest an expansion of the star during
strong accretion events whereas the luminosity spread implies a contraction of
the accreting objects, decreasing their radiating surface. In this paper, we
present a global scenario based on calculations coupling episodic accretion
histories derived from numerical simulations of collapsing cloud prestellar
cores of various masses and subsequent protostar evolution. Our calculations
show that, assuming an initial protostar mass \mi \sim 1\,\mjup, typical of
the second Larson's core, both the luminosity spread in the HRD and the
inferred properties of Fu Ori events (mass, radius, accretion rate) can be
explained by this scenario, providing two conditions. First, there must be some
variation within the fraction of accretion energy absorbed by the protostar
during the accretion process. Second the range of this variation should
increase with increasing accretion burst intensity, and thus with the initial
core mass and final star mass. The numerical hydrodynamics simulations of
collapsing cloud prestellar cores indeed show that the intensity of the
accretion bursts correlates with the mass and initial angular momentum of the
prestellar core. Massive prestellar cores with high initial angular momentum
are found to produce intense bursts characteristic of Fu Ori like events. Our
results thus suggest a link between the burst intensities and the fraction of
accretion energy absorbed by the protostar, with some threshold in the
accretion rate, of the order of 10^{-5}\msolyr, delimitating the transition
from "cold" to "hot" accretion. [Abridged]Comment: 23 pages, 5 figures, ApJ accepte
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