698 research outputs found
Constraining the evolutionary history of Newton's constant with gravitational wave observations
Space-borne gravitational wave detectors, such as the proposed Laser
Interferometer Space Antenna, are expected to observe black hole coalescences
to high redshift and with large signal-to-noise ratios, rendering their
gravitational waves ideal probes of fundamental physics. The promotion of
Newton's constant to a time-function introduces modifications to the binary's
binding energy and the gravitational wave luminosity, leading to corrections in
the chirping frequency. Such corrections propagate into the response function
and, given a gravitational wave observation, they allow for constraints on the
first time-derivative of Newton's constant at the time of merger. We find that
space-borne detectors could indeed place interesting constraints on this
quantity as a function of sky position and redshift, providing a
{\emph{constraint map}} over the entire range of redshifts where binary black
hole mergers are expected to occur. A LISA observation of an equal-mass
inspiral event with total redshifted mass of 10^5 solar masses for three years
should be able to measure at the time of merger to better than
10^(-11)/yr.Comment: 11 pages, 2 figures, replaced with version accepted for publication
in Phys. Rev. D
Horizon Formation in High-Energy Particles Collision
We investigate a classical formation of a trapped surface in 4-dimensional
flat space-time in a process of a non-head-on collision of two high-energy
particles which are treated as Aichelburg-Sexl shock waves. From the condition
of the horizon volume local maximality an equation for the trapped surface is
deduced. Using a known solution on the shocks we find a time-dependent solution
describing the trapped surface between the shocks. We analyze the horizon
appearance and evolution. Obtained results may describe qualitatively the
horizon formation in higher dimensional space-time.Comment: Latex2e, 8 pages, 6 figures, references adde
Simulation of Asymptotically AdS5 Spacetimes with a Generalized Harmonic Evolution Scheme
Motivated by the gauge/gravity duality, we introduce a numerical scheme based
on generalized harmonic evolution to solve the Einstein field equations on
asymptotically anti-de Sitter (AdS) spacetimes. We work in global AdS5, which
can be described by the (t,r,\chi,\theta,\phi) spherical coordinates adapted to
the R{\times}S3 boundary. We focus on solutions that preserve an SO(3) symmetry
that acts to rotate the 2-spheres parametrized by \theta,\phi. In the boundary
conformal field theory (CFT), the way in which this symmetry manifests itself
hinges on the way we choose to embed Minkowski space in R{\times}S3. We present
results from an ongoing study of prompt black hole formation via scalar field
collapse, and explore the subsequent quasi-normal ringdown. Beginning with
initial data characterized by highly distorted apparent horizon geometries, the
metrics quickly evolve, via quasi-normal ringdown, to equilibrium static black
hole solutions at late times. The lowest angular number quasi-normal modes are
consistent with the linear modes previously found in perturbative studies,
whereas the higher angular modes are a combination of linear modes and of
harmonics arising from non-linear mode-coupling. We extract the stress energy
tensor of the dual CFT on the boundary, and find that despite being highly
inhomogeneous initially, it nevertheless evolves from the outset in a manner
that is consistent with a thermalized N=4 SYM fluid. As a first step towards
closer contact with relativistic heavy ion collision physics, we map this
solution to a Minkowski piece of the R{\times}S3 boundary, and obtain a
corresponding fluid flow in Minkowski space
Aperiodic optical variability of intermediate polars - cataclysmic variables with truncated accretion disks
We study the power spectra of the variability of seven intermediate polars
containing magnetized asynchronous accreting white dwarfs, XSS J00564+4548,IGR
J00234+6141, DO Dra, V1223 Sgr, IGR J15094-6649, IGR J16500-3307 and IGR
J17195-4100, in the optical band and demonstrate that their variability can be
well described by a model based on fluctuations propagating in a truncated
accretion disk. The power spectra have breaks at Fourier frequencies, which we
associate with the Keplerian frequency of the disk at the boundary of the white
dwarfs' magnetospheres. We propose that the properties of the optical power
spectra can be used to deduce the geometry of the inner parts of the accretion
disk, in particular: 1) truncation radii of the magnetically disrupted
accretion disks in intermediate polars, 2) the truncation radii of the
accretion disk in quiescent states of dwarf novaeComment: Accepted for publication in A&
The Relationship Between X-ray Luminosity and Duty Cycle for Dwarf Novae and their Specific Frequency in the Inner Galaxy
We measure the duty cycles for an existing sample of well observed, nearby
dwarf novae using data from AAVSO, and present a quantitative empirical
relation between the duty cycle of dwarf novae outbursts and the X-ray
luminosity of the system in quiescence. We have found that , where
DC stands for duty cycle. We note that there is intrinsic scatter in this
relation greater than what is expected from purely statistical errors. Using
the dwarf nova X-ray luminosity functions from \citet{Pretorius12} and
\citet{Byckling10}, we compare this relation to the number of dwarf novae in
the Galactic Bulge Survey which were identified through optical outbursts
during an 8-day long monitoring campaign. We find a specific frequency of X-ray
bright () Cataclysmic Variables undergoing
Dwarf Novae outbursts in the direction of the Galactic Bulge of
. Such a specific frequency would give
a Solar neighborhood space density of long period CVs of
pc. We advocate the use of specific
frequency in future work, given that projects like LSST will detect DNe well
outside the distance range over which .Comment: 9 pagers, 4 figures Accepted for publication in MNRA
Semianalytical estimates of scattering thresholds and gravitational radiation in ultrarelativistic black hole encounters
Ultrarelativistic collisions of black holes are ideal gedanken experiments to
study the nonlinearities of general relativity. In this paper we use
semianalytical tools to better understand the nature of these collisions and
the emitted gravitational radiation. We explain many features of the energy
spectra extracted from numerical relativity simulations using two complementary
semianalytical calculations. In the first calculation we estimate the radiation
by a "zero-frequency limit" analysis of the collision of two point particles
with finite impact parameter. In the second calculation we replace one of the
black holes by a point particle plunging with arbitrary energy and impact
parameter into a Schwarzschild black hole, and we explore the multipolar
structure of the radiation paying particular attention to the near-critical
regime. We also use a geodesic analogy to provide qualitative estimates of the
dependence of the scattering threshold on the black hole spin and on the
dimensionality of the spacetime.Comment: 29 pages, 19 figure, 6 tables, minor changes to match version in
press in Phys.Rev.
Building capacity and developing human capital: an exploration of curriculum development in ICT programmes at South African universities
There is a growing consensus on the potential for information and communications technologies (ICTs) to support socio-economic development in sub-Saharan Africa (SSA). Universities as providers of higher education are critical to developing SSA countries by, amongst others, empowering the region to develop appropriate ICT solutions for local challenges. The purpose of this paper is to describe and discuss how contextual realities influence the development of ICT programme curricula at public universities in South Africa, a developing country in SSA, in order to meet the demand for ICT skills. After a brief discussion of the factors that influence the content and delivery in education systems an overview of the South African public higher education context is provided. Against this background the content and delivery of ICT programme offerings and the rationale behind the programmes at four South African public universities are presented in case study format and concludes with a discussion of the case studies. The paper may be of value to academic departments as examples of how other departments are responding and adjusting their offerings, to government departments and policy makers by engendering a better understanding of the impact of policy on programme development and to industry by illustrating the diverse stakeholders in higher education and academia’s responsiveness to these diverse requirements
AMR, stability and higher accuracy
Efforts to achieve better accuracy in numerical relativity have so far
focused either on implementing second order accurate adaptive mesh refinement
or on defining higher order accurate differences and update schemes. Here, we
argue for the combination, that is a higher order accurate adaptive scheme.
This combines the power that adaptive gridding techniques provide to resolve
fine scales (in addition to a more efficient use of resources) together with
the higher accuracy furnished by higher order schemes when the solution is
adequately resolved. To define a convenient higher order adaptive mesh
refinement scheme, we discuss a few different modifications of the standard,
second order accurate approach of Berger and Oliger. Applying each of these
methods to a simple model problem, we find these options have unstable modes.
However, a novel approach to dealing with the grid boundaries introduced by the
adaptivity appears stable and quite promising for the use of high order
operators within an adaptive framework
Gravitational perturbations of Schwarzschild spacetime at null infinity and the hyperboloidal initial value problem
We study gravitational perturbations of Schwarzschild spacetime by solving a
hyperboloidal initial value problem for the Bardeen-Press equation.
Compactification along hyperboloidal surfaces in a scri-fixing gauge allows us
to have access to the gravitational waveform at null infinity in a general
setup. We argue that this hyperboloidal approach leads to a more accurate and
efficient calculation of the radiation signal than the common approach where a
timelike outer boundary is introduced. The method can be generalized to study
perturbations of Kerr spacetime using the Teukolsky equation.Comment: 14 pages, 9 figure
The intriguing nature of the high energy gamma ray source XSSJ12270-4859
The nature of the hard X-ray source XSSJ12270-4859 is still unclear though it
was claimed to be a magnetic Cataclysmic Variable. We here present a broad-band
X-ray and gamma ray study based on a recent XMM-Newton observation and archival
INTEGRAL and RXTE data. From the Fermi/LAT 1-year point source catalogue, we
tentatively associate XSSJ12270-4859 with 1FGLJ1227.9-4852, a source of high
energy gamma rays with emission up to 10GeV. We complement the study with UV
photometry from XMM-Newton and ground-based optical and near-IR photometry. The
X-ray emission is highly variable showing flares and intensity dips. The X-ray
flares consist of flare-dip pairs. Flares are also detected in the UV range but
not the dips. Aperiodic dipping behaviour is also observed during X-ray
quiescence but not in the UV. The 0.2-100keV spectrum is featureless and
described by a power law model with Gamma=1.7. The 100MeV-10GeV spectrum is
instead represented by a power law index of 2.45. The luminosity ratio between
0.1-100GeV and 0.2--100keV is ~0.8, hence the GeV emission is a significant
component of the total energy output. Furthermore, the X-ray spectrum does not
greatly change during flares, quiescence and the dips seen in quiescence but it
hardens during the post-flare dips. Optical photometry reveals a period of
4.32hr likely related to the binary orbit. Near-IR, possibly ellipsoidal,
variations are detected. Large amplitude variability on shorter (tens mins)
timescales are found to be non-periodic. The observed variability at all
wavelengths and the spectral characteristics strongly favour a low-mass
atypical low-luminosity X-ray binary and are against a Cataclysmic Variable
nature. The association with a Fermi/LAT high energy gamma ray source further
strengths this interpretation.Comment: 12 pages, 11 figures, 3 tables; Accepted for publication in Astronomy
& Astrophysics Main Journ
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