412 research outputs found
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
High-Frequency and Type-C QPOs from Oscillating, Precessing Hot, Thick Flow
Motivated by recent studies showing an apparent correlation between the
high-frequency quasi-periodic oscillations (QPOs) and the low-frequency, type-C
QPO in low-mass, black hole X-ray binaries (LMXBs), we explore a model that
explains all three QPOs in terms of an oscillating, precessing hot flow in the
truncated-disk geometry. Our model favors attributing the two high-frequency
QPOs, often occurring in a near 3:2 frequency ratio, to the breathing and
vertical epicyclic frequency modes of the hot, thick flow, although we can not
rule out the Keplerian and m=-1 radial epicyclic modes. In either case, the
type-C QPO is attributed to precession. The correlation of the QPOs comes from
the fact that all three frequencies are associated with the same geometrical
structure. While the exact QPO frequencies are sensitive to the black hole mass
and spin, their evolution over the course of an outburst is mainly tied to the
truncation radius between the geometrically thin, optically thick disk and the
inner, hot flow. We show that, in the case of the LMXB GRO J1655-40, this model
can explain the one simultaneous observation of all three QPOs and that an
extrapolation of the model appears to match lower frequency observations where
only two of the three components are seen. Thus, this model may be able to
unify multiple QPO observations using the properties of a single, simple,
geometrical model.Comment: 7 pages, 4 figures, accepted 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
Quasi-Periodic Oscillations from Magnetorotational Turbulence
Quasi-periodic oscillations (QPOs) in the X-ray lightcurves of accreting
neutron star and black hole binaries have been widely interpreted as being due
to standing wave modes in accretion disks. These disks are thought to be highly
turbulent due to the magnetorotational instability (MRI). We study wave
excitation by MRI turbulence in the shearing box geometry. We demonstrate that
axisymmetric sound waves and radial epicyclic motions driven by MRI turbulence
give rise to narrow, distinct peaks in the temporal power spectrum. Inertial
waves, on the other hand, do not give rise to distinct peaks which rise
significantly above the continuum noise spectrum set by MRI turbulence, even
when the fluid motions are projected onto the eigenfunctions of the modes. This
is a serious problem for QPO models based on inertial waves.Comment: 4 pages, 2 figures. submitted to ap
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
Excitation of Trapped Waves in Simulations of Tilted Black Hole Accretion Disks with Magnetorotational Turbulence
We analyze the time dependence of fluid variables in general relativistic,
magnetohydrodynamic simulations of accretion flows onto a black hole with
dimensionless spin parameter a/M=0.9. We consider both the case where the
angular momentum of the accretion material is aligned with the black hole spin
axis (an untilted flow) and where it is misaligned by 15 degrees (a tilted
flow). In comparison to the untilted simulation, the tilted simulation exhibits
a clear excess of inertial variability, that is, variability at frequencies
below the local radial epicyclic frequency. We further study the radial
structure of this inertial-like power by focusing on a radially extended band
at 118 (M/10Msol)^-1 Hz found in each of the three analyzed fluid variables.
The three dimensional density structure at this frequency suggests that the
power is a composite oscillation whose dominant components are an over dense
clump corotating with the background flow, a low order inertial wave, and a low
order inertial-acoustic wave. Our results provide preliminary confirmation of
earlier suggestions that disk tilt can be an important excitation mechanism for
inertial waves.Comment: 8 Pages, 6 Figures, accepted for publication in Ap
Broad-band continuum and variability of NGC 5548
We analyze a composite broad-band optical/UV/Xgamma-ray spectrum of the
Seyfert 1 galaxy NGC 5548. The spectrum consists of an average of simultaneous
optical/IUE/Ginga observations accompanied by ROSAT and GRO/OSSE data from
non-simultaneous observations. We show that the broad-band continuum is
inconsistent with simple disk models extending to the soft X-rays. Instead, the
soft-excess is well described by optically thick, low temperature, thermal
Comptonization which may dominate the entire big blue bump. This might explain
the observed tight UV/soft X-ray variability correlation and absence of a Lyman
edge in this object. However, the plasma parameters inferred by the spectrum
need stratification in optical depth and/or temperature to prevent physical
inconsistency. The optical/UV/soft X-ray component contributes about half of
the total source flux. The spectral variations of the soft-excess are
consistent with that of the UV and argue that the components are closely
related. The overall pattern of spectral variability suggests variations of the
source geometry, and shows the optical/UV/soft X-ray component to be harder
when brighter, while the hard X-ray component is softer when brighter.Comment: 5 pages, 3 figures, LaTex, using AIP macro, to appear in Proceedings
of "4th Compton Symposium", April 27-30, 1997, Williamsburg, Virginia, US
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