233 research outputs found
Magnetic pressure support and accretion disk spectra
Stellar atmosphere models of ionized accretion disks have generally neglected
the contribution of magnetic fields to the vertical hydrostatic support,
although magnetic fields are widely believed to play a critical role in the
transport of angular momentum. Simulations of magnetorotational turbulence in a
vertically stratified shearing box geometry show that magnetic pressure support
can be dominant in the upper layers of the disk. We present calculations of
accretion disk spectra that include this magnetic pressure support, as well as
a vertical dissipation profile based on simulation. Magnetic pressure support
generically produces a more vertically extended disk atmosphere with a larger
density scale height. This acts to harden the spectrum compared to models that
neglect magnetic pressure support. We estimate the significance of this effect
on disk-integrated spectra by calculating an illustrative disk model for a
stellar mass black hole, assuming that similar magnetic pressure support exists
at all radii.Comment: submitted to Ap
Oscillation modes of relativistic slender tori
Accretion flows with pressure gradients permit the existence of standing
waves which may be responsible for observed quasi-periodic oscillations (QPO's)
in X-ray binaries. We present a comprehensive treatment of the linear modes of
a hydrodynamic, non-self-gravitating, polytropic slender torus, with arbitrary
specific angular momentum distribution, orbiting in an arbitrary axisymmetric
spacetime with reflection symmetry. We discuss the physical nature of the
modes, present general analytic expressions and illustrations for those which
are low order, and show that they can be excited in numerical simulations of
relativistic tori. The mode oscillation spectrum simplifies dramatically for
near Keplerian angular momentum distributions, which appear to be generic in
global simulations of the magnetorotational instability. We discuss our results
in light of observations of high frequency QPO's, and point out the existence
of a new pair of modes which can be in an approximate 3:2 ratio for arbitrary
black hole spins and angular momentum distributions, provided the torus is
radiation pressure dominated. This mode pair consists of the axisymmetric
vertical epicyclic mode and the lowest order axisymmetric breathing mode.Comment: submitted to MNRA
Neutron starquakes and the nature of gamma-ray bursts
The possibility that gamma-ray bursts originate from quakes deep in the solid crust of a neutron star is investigated. Seismic waves are radiated if shear stress is relieved by brittle fracture. However they cannot propagate directly to the surface but are temporarily trapped below a reflecting layer. The shaking of the stellar surface couples the seismic waves to Alfven waves which propagate out into the magnetosphere. The crust-magnetosphere transmission coefficient strongly increases with wave frequency and magnetic field strength. Alfven wave luminosities sufficient to power galactic gamma-ray bursts are possible if magnetic fields greater than 100 billion G cover at least part of the stellar surface. As the Alfven waves propagate out into the low density magnetosphere, they become increasingly charge starved, thereby accelerating particles to relativistic energies
Neutron starquake models for gamma-ray bursts
We assess neutron starquake models for γ-ray bursts. The elastic energy the crust can store is sufficient to account for that radiated in a single burst, but it is insufficient to supply the ≳ 10^6 bursts each star produces over its lifetime, and so it must be replenished. Seismic waves are radiated if shear stress is relieved by brittle fracture. However they cannot propagate directly to the surface but are temporarily trapped below a reflecting layer. Between the reflecting layer and the surface the displacement amplitude of the wave is nearly constant and the strain is very small. At low frequencies, ≾ 10^4 Hz, the reflection is associated with an evanescent zone. At high frequencies, ≳ 10^4 Hz, the reflection occurs where the magnetic field stress starts to dominate the crustal rigidity. The shaking of the stellar surface couples the seismic waves to Alfén waves which propagate out into the magnetosphere. At low frequencies, the coupling coefficient, T, is proportional to the square of the magnetic field, B, and increases with the seventh power of the wave frequency, v. At high frequencies, T is proportional to B^(4/7)v^(3/7). Alfvén wave luminosities sufficient to power Galactic γ-ray bursts are possible if magnetic fields ≳ 10^(11) G cover at least part of the stellar surface. The conversion of Alfvén waves into γ-rays may occur if the waves are charge-starved or if their amplitudes approach that of the background magnetic field
Epicyclic oscillations of non-slender fluid tori around Kerr black holes
Considering epicyclic oscillations of pressure-supported perfect fluid tori
orbiting Kerr black holes we examine non-geodesic (pressure) effects on the
epicyclic modes properties. Using a perturbation method we derive fully general
relativistic formulas for eigenfunctions and eigenfrequencies of the radial and
vertical epicyclic modes of a slightly non-slender, constant specific angular
momentum torus up to second-order accuracy with respect to the torus thickness.
The behaviour of the axisymmetric and lowest-order () non-axisymmetric
epicyclic modes is investigated. For an arbitrary black hole spin we find that,
in comparison with the (axisymmetric) epicyclic frequencies of free test
particles, non-slender tori receive negative pressure corrections and exhibit
thus lower frequencies. Our findings are in qualitative agreement with the
results of a recent pseudo-Newtonian study of analogous problem defined within
the Paczy{\'n}ski-Wiita potential. Implications of our results on the
high-frequency QPO models dealing with epicyclic oscillations are addressed.Comment: 24 pages, 8 figure
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