512 research outputs found
Epicyclic oscillations of fluid bodies Paper II. Strong gravity
Fluids in external gravity may oscillate with frequencies characteristic of
the epicyclic motions of test particles. We explicitly demonstrate that global
oscillations of a slender, perfect fluid torus around a Kerr black hole admit
incompressible vertical and radial epicyclic modes. Our results may be directly
relevant to one of the most puzzling astrophysical phenomena -- high (hundreds
of hertz) frequency quasiperiodic oscillations (QPOs) detected in X-ray fluxes
from several black hole sources. Such QPOs are pairs of stable frequencies in
the 3/2 ratio. It seems that they originate a few gravitational radii away from
the black hole and thus observations of them have the potential to become an
accurate probe of super-strong gravity.Comment: submitted to Classical and Quantum Gravit
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 stars accreting the ISM: Are they fast or slow objects ?
Old neutron stars (ONSs) which have radiated away their internal and
rotational energy may still shine if accreting the interstellar medium. Rather
stringent limits from the analysis of ROSAT surveys indicate that most
optimistic predictions on ONSs observability are in excess of a factor as large
as . Here we explore two possible evolutionary scenarios that may
account for the paucity of ONSs. In the first it is assumed that the ONS
population is not too fast () and that magnetic field decay
guides the evolution. In the second, NSs move with high speed ( km
s) and preserve their magnetic field at birth. We find that according to
the former scenario most ONSs are now in the propeller phase, while in the
latter nearly all ONSs are silent, dead pulsars.Comment: 5 pages including 2 postscript figures, to appear in the proceedings
of Rome BeppoSax-RossiXTE meetin
Design and qualification of the SEU/TD Radiation Monitor chip
This report describes the design, fabrication, and testing of the Single-Event Upset/Total Dose (SEU/TD) Radiation Monitor chip. The Radiation Monitor is scheduled to fly on the Mid-Course Space Experiment Satellite (MSX). The Radiation Monitor chip consists of a custom-designed 4-bit SRAM for heavy ion detection and three MOSFET's for monitoring total dose. In addition the Radiation Monitor chip was tested along with three diagnostic chips: the processor monitor and the reliability and fault chips. These chips revealed the quality of the CMOS fabrication process. The SEU/TD Radiation Monitor chip had an initial functional yield of 94.6 percent. Forty-three (43) SEU SRAM's and 14 Total Dose MOSFET's passed the hermeticity and final electrical tests and were delivered to LL
The Kozai Mechanism and the Evolution of Binary Supermassive Black Holes
We consider the dynamical evolution of bound, hierarchical triples of supermassive black holes that might be formed in the nuclei of galaxies undergoing sequential mergers. The tidal force of the outer black hole on the inner binary produces eccentricity oscillations through the Kozai mechanism, and this can substantially reduce the gravitational wave merger time of the inner binary. We numerically calculate the merger time for a wide range of initial conditions and black hole mass ratios, including the effects of octupole interactions in the triple as well as general relativistic periastron precession in the inner binary. The semimajor axes and the mutual inclination of the inner and outer binaries are the most important factors affecting the merger time. We find that for a random distribution of inclination angles and approximately equal mass black holes, it is possible to reduce the merger time of a near circular inner binary by more than a factor of ten in over thirty percent of all cases. We estimate that a typical exterior quadrupole moment from surrounding matter in the galaxy may also be sufficient to excite eccentricity oscillations in supermassive black hole binaries, and also accelerate black hole mergers
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
Ambipolar Diffusion in the Magnetorotational Instability
The effects of ambipolar diffusion on the linear stability of weakly ionised
accretion discs are examined. Earlier work on this topic has focused on axial
magnetic fields and perturbation wavenumbers. We consider here more general
field and wavenumber geometries, and find that qualitatively new results are
obtained. Provided a radial wavenumber and azimuthal field are present along
with their axial counterparts, ambipolar diffusion will always be
destabilising, with unstable local modes appearing at well-defined wavenumber
bands. The wavenumber corresponding to the maximum growth rate need not, in
general, lie along the vertical axis. Growth rates become small relative to the
local angular velocity when the ion-neutral collision time exceeds the orbital
time. In common with Hall electromotive forces, ambipolar diffusion
destabilises both positive and negative angular velocity gradients. In at least
some cases, therefore, uniformly rotating molecular cloud cores may reflect the
marginally stable state of the ambipolar magnetorotational instability.Comment: Submitted to MN, 6 pages, 3 figs, MN style file v2.
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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