2,521 research outputs found
Oscillations of Thick Accretion Discs Around Black Holes
We present a numerical study of the response of a thick accretion disc to a
localized, external perturbation with the aim of exciting internal modes of
oscillation. We find that the perturbations efficiently excite global modes
recently identified as acoustic p--modes, and closely related to the epicyclic
oscillations of test particles. The two strongest modes occur at
eigenfrequencies which are in a 3:2 ratio. We have assumed a constant specific
angular momentum distribution within the disc. Our models are in principle
scale--free and can be used to simulate accretion tori around stellar or super
massive black holes.Comment: 4 pages, 4 figures, accepted for publication as a letter in the
Monthly Notices of the Royal Astronomical Societ
Oscillations of Thick Accretion Discs Around Black Holes - II
We present a numerical study of the global modes of oscillation of thick
accretion discs around black holes. We have previously studied the case of
constant distributions of specific angular momentum. In this second paper, we
investigate (i) how the size of the disc affects the oscillation
eigenfrequencies, and (ii) the effect of power-law distributions of angular
momentum on the oscillations. In particular, we compare the oscillations of the
disc with the epicyclic eigenfrequencies of a test particle with different
angular momentum distributions orbiting around the central object. We find that
there is a frequency shift away from the epicyclic eigenfrequency of the test
particle to lower values as the size of the tori is increased. We have also
studied the response of a thick accretion disc to a localized external
perturbation using non constant specific angular momentum distributions within
the disc. We find that in this case it is also possible (as reported previously
for constant angular momentum distributions) to efficiently excite internal
modes of oscillation. In fact we show here that the local perturbations excite
global oscillations (acoustic p modes) closely related to the epicyclic
oscillations of test particles. Our results are particularly relevant in the
context of low mass X-ray binaries and microquasars, and the high frequency
Quasi-Periodic Oscillations (QPOs) observed in them. Our computations make use
of a Smooth Particle Hydrodynamics (SPH) code in azimuthal symmetry, and use a
gravitational potential that mimics the effects of strong gravity.Comment: 10 pages, 8 figures, accepted for publication as a paper in the
Monthly Notices of the Royal Astronomical Societ
Past and future gauge in numerical relativity
Numerical relativity describes a discrete initial value problem for general
relativity. A choice of gauge involves slicing space-time into space-like
hypersurfaces. This introduces past and future gauge relative to the
hypersurface of present time. Here, we propose solving the discretized Einstein
equations with a choice of gauge in the future and a dynamical gauge in the
past. The method is illustrated on a polarized Gowdy wave.Comment: To appear in Class Quantum Grav, Let
Scattering Lens Resolves sub-100 nm Structures with Visible Light
The smallest structures that conventional lenses are able to optically
resolve are of the order of 200 nm. We introduce a new type of lens that
exploits multiple scattering of light to generate a scanning nano-sized optical
focus. With an experimental realization of this lens in gallium phosphide we
have succeeded to image gold nanoparticles at 97 nm optical resolution. Our
work is the first lens that provides a resolution in the nanometer regime at
visible wavelengths.Comment: 4 pages, 3 figure
A new method of preparing a rotating ring-disc electrode for the study of carbon supported catalysts
Entropic force in black hole binaries and its Newtonian limits
We give an exact solution for the static force between two black holes at the
turning points in their binary motion. The results are derived by Gibbs'
principle and the Bekenstein-Hawking entropy applied to the apparent horizon
surfaces in time-symmetric initial data. New power laws are derived for the
entropy jump in mergers, while Newton's law is shown to derive from a new
adiabatic variational principle for the Hilbert action in the presence of
apparent horizon surfaces. In this approach, entropy is strictly monotonic such
that gravity is attractive for all separations including mergers, and the
Bekenstein entropy bound is satisfied also at arbitrarily large separations,
where gravity reduces to Newton's law. The latter is generalized to point
particles in the Newtonian limit by application of Gibbs' principle to
world-lines crossing light cones.Comment: Accepted for publication in Phys. Rev.
Non-Imaging Speckle Interferometry forHigh Speed Nanometer-Scale Position Detection
We experimentally demonstrate a non-imaging approach to displacement
measurement for complex scattering materials. By spatially controlling the wave
front of the light that incidents on the material we concentrate the scattered
light in a focus on a designated position. This wave front acts as an unique
optical fingerprint that enables precise position detection of the illuminated
material by simply measuring the intensity in the focus. By combining two
optical fingerprints we demonstrate position detection along one dimension with
a displacement resolution of 2.1 nm. As our approach does not require an image
of the scattered field, it is possible to employ fast non-imaging detectors to
enable high-speed position detection of scattering materials.Comment: 4 pages, 3 figure
Spatial amplitude and phase modulation using commercial twisted nematic LCDs
We present a method for full spatial phase and amplitude control of a laser
beam using a twisted nematic liquid crystal display combined with a spatial
filter. By spatial filtering we combine four neighboring pixels into one
superpixel. At each superpixel we are able to independently modulate the phase
and the amplitude of light. We demonstrate experimentally the independent phase
and amplitude modulation using this novel technique. Our technique does not
impose special requirements on the spatial light modulator and allows precise
control of fields even with imperfect modulators.Comment: 10 pages, 6 figure
Gravitational radiation from gamma-ray bursts as observational opportunities for LIGO and VIRGO
Gamma-ray bursts are believed to originate in core-collapse of massive stars.
This produces an active nucleus containing a rapidly rotating Kerr black hole
surrounded by a uniformly magnetized torus represented by two counter-oriented
current rings. We quantify black hole spin-interactions with the torus and
charged particles along open magnetic flux-tubes subtended by the event
horizon. A major output of Egw=4e53 erg is radiated in gravitational waves of
frequency fgw=500 Hz by a quadrupole mass-moment in the torus. Consistent with
GRB-SNe, we find (i) Ts=90s (tens of s, Kouveliotou et al. 1993), (ii)
aspherical SNe of kinetic energy Esn=2e51 erg (2e51 erg in SN1998bw, Hoeflich
et al. 1999) and (iii) GRB-energies Egamma=2e50 erg (3e50erg in Frail et al.
2001). GRB-SNe occur perhaps about once a year within D=100Mpc. Correlating
LIGO/Virgo detectors enables searches for nearby events and their spectral
closure density 6e-9 around 250Hz in the stochastic background radiation in
gravitational waves. At current sensitivity, LIGO-Hanford may place an upper
bound around 150MSolar in GRB030329. Detection of Egw thus provides a method
for identifying Kerr black holes by calorimetry.Comment: to appear in PRD, 49
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