2,415 research outputs found
Dynamics of thick discs around Schwarzschild-de Sitter black holes
We consider the effects of a cosmological constant on the dynamics of
constant angular momentum discs orbiting Schwarzschild-de Sitter black holes.
The motivation behind this study is to investigate whether the presence of a
radial force contrasting the black hole's gravitational attraction can
influence the occurrence of the runaway instability, a robust feature of the
dynamics of constant angular momentum tori in Schwarzschild and Kerr
spacetimes. In addition to the inner cusp near the black hole horizon through
which matter can accrete onto the black hole, in fact, a positive cosmological
constant introduces also an outer cusp through which matter can leave the torus
without accreting onto the black hole. To assess the impact of this outflow on
the development of the instability we have performed time-dependent and
axisymmetric hydrodynamical simulations of equilibrium initial configurations
in a sequence of background spacetimes of Schwarzschild-de Sitter black holes
with increasing masses. The simulations have been performed with an unrealistic
value for the cosmological constant which, however, yields sufficiently small
discs to be resolved accurately on numerical grids and thus provides a first
qualitative picture of the dynamics. The calculations, carried out for a wide
range of initial conditions, show that the mass-loss from the outer cusp can
have a considerable impact on the instability, with the latter being rapidly
suppressed if the outflow is large enough.Comment: 12 pages; A&A, in pres
Humidity contribution to C_n^2 over a 600m pathlength in a tropical marine environment
We present new optical turbulence structure parameter measurements, C_n^2,
over sea water between La Parguera and Magueyes Island (17.6N 67W) on the
southwest coast of Puerto Rico. The 600 meter horizontal paths were located
approximately 1.5 m and 10 m above sea level. No data of this type has ever
been made available in the literature. Based on the data, we show that the
C_n^2 measurements are about 7 times less compared to equivalent land data.
This strong evidence reinforces our previous argument that humidity must be
accounted for to better ascertain the near surface atmospheric turbulence
effects, which current visible / near infrared C_n^2 bulk models fail to do. We
also explore the generalised fractal dimension of this littoral data and
compare it to our reference land data. We find cases that exhibit monofractal
characteristics, that is to say, the effect of rising temperatures during the
daylight hours upon turbulence are counterbalanced by humidity, leading to a
single characteristic scale for the measurements. In other words, significant
moisture changes in the measurement volume cancels optical turbulence increases
due to temperature rises. Figures available as JPG only.Comment: 7 pages, 4 figures, 1 table, SPIE Photonics West 2007, paper 6457B-2
Turbulence Time Series Data Hole Filling using Karhunen-Loeve and ARIMA methods
Measurements of optical turbulence time series data using unattended
instruments over long time intervals inevitably lead to data drop-outs or
degraded signals. We present a comparison of methods using both Principal
Component Analysis, which is also known as the Karhunen--Loeve decomposition,
and ARIMA that seek to correct for these event-induced and mechanically-induced
signal drop-outs and degradations. We report on the quality of the correction
by examining the Intrinsic Mode Functions generated by Empirical Mode
Decomposition. The data studied are optical turbulence parameter time series
from a commercial long path length optical anemometer/scintillometer, measured
over several hundred metres in outdoor environments.Comment: 8 pages, 9 figures, submitted to ICOLAD 2007, City University,
London, U
On the development of QPOs in Bondi-Hoyle accretion flows
The numerical investigation of Bondi-Hoyle accretion onto a moving black hole
has a long history, both in Newtonian and in general-relativistic physics. By
performing new two-dimensional and general-relativistic simulations onto a
rotating black hole, we point out a novel feature, namely, that quasi-periodic
oscillations (QPOs) are naturally produced in the shock cone that develops in
the downstream part of the flow. Because the shock cone in the downstream part
of the flow acts as a cavity trapping pressure perturbations, modes with
frequencies in the integer ratios 2:1 and 3:1 are easily produced. The
frequencies of these modes depend on the black-hole spin and on the properties
of the flow, and scale linearly with the inverse of the black-hole mass. Our
results may be relevant for explaining the detection of QPOs in Sagittarius A*,
once such detection is confirmed by further observations. Finally, we report on
the development of the flip-flop instability, which can affect the shock cone
under suitable conditions; such an instability has been discussed before in
Newtonian simulations but was never found in a relativistic regime.Comment: 11 pages, 7 figure
Applying the Hilbert--Huang Decomposition to Horizontal Light Propagation C_n^2 data
The Hilbert Huang Transform is a new technique for the analysis of
non--stationary signals. It comprises two distinct parts: Empirical Mode
Decomposition (EMD) and the Hilbert Transform of each of the modes found from
the first step to produce a Hilbert Spectrum. The EMD is an adaptive
decomposition of the data, which results in the extraction of Intrinsic Mode
Functions (IMFs). We discuss the application of the EMD to the calibration of
two optical scintillometers that have been used to measure C_n^2 over
horizontal paths on a building rooftop, and discuss the advantage of using the
Marginal Hilbert Spectrum over the traditional Fourier Power Spectrum.Comment: 9 pages, 11 figures, proc. SPIE 626
Topological Inflation in Dual Superstring Models
We study the possibility of obtaining inflationary solutions from S-dual
superstring potentials. We find, in particular, that such solutions occur at
the core of domain walls separating degenerate minima whose positions differ by
modular transformations.Comment: 12 pages, uuencoded gzipped tar format, Latex, 2 figure
Dynamics of oscillating magnetized relativistic tori around a Schwarzschild black hole
We present a comprehensive numerical study of the dynamics of magnetized relativistic axisymmetric tori orbiting in the background spacetime of a Schwarzschild black hole. The tori are modeled as having a purely toroidal magnetic field and a constant distribution of the specific angular momentum. Following previous investigations of tori in a purely hydrodynamical context, the dynamics of these objects has been studied upon the introduction of a perturbation which, for the values of the magnetic field considered here, triggers quasi-periodic oscillations (QPOs) lasting tens of orbital periods. As in the hydrodynamical case, the spectral distribution of the eigenfrequencies shows the presence of a fundamental p-mode and of overtones in a harmonic ratio: 2 : 3 : .... We comment on the implications of these results on the phenomenology observed for the QPOs in low-mass X-ray binaries containing a black hole candidate
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