2,404 research outputs found
Interpreting the High Frequency QPO Power Spectra of Accreting Black Holes
In the context of a relativistic hot spot model, we investigate different
physical mechanisms to explain the behavior of quasi-periodic oscillations
(QPOs) from accreting black holes. The locations and amplitudes of the QPO
peaks are determined by the ray-tracing calculations presented in Schnittman &
Bertschinger (2004a): the black hole mass and angular momentum give the
geodesic coordinate frequencies, while the disk inclination and the hot spot
size, shape, and overbrightness give the amplitudes of the different peaks. In
this paper additional features are added to the existing model to explain the
broadening of the QPO peaks as well as the damping of higher frequency
harmonics in the power spectrum. We present a number of analytic results that
closely agree with more detailed numerical calculations. Four primary pieces
are developed: the addition of multiple hot spots with random phases, a finite
width in the distribution of geodesic orbits, Poisson sampling of the detected
photons, and the scattering of photons from the hot spot through a corona of
hot electrons around the black hole. Finally, the complete model is used to fit
the observed power spectra of both type A and type B QPOs seen in XTE
J1550-564, giving confidence limits on each of the model parameters.Comment: 30 pages, 5 figures, submitted to Ap
Panoramic optical and near-infrared SETI instrument: optical and structural design concepts
We propose a novel instrument design to greatly expand the current optical
and near-infrared SETI search parameter space by monitoring the entire
observable sky during all observable time. This instrument is aimed to search
for technosignatures by means of detecting nano- to micro-second light pulses
that could have been emitted, for instance, for the purpose of interstellar
communications or energy transfer. We present an instrument conceptual design
based upon an assembly of 198 refracting 0.5-m telescopes tessellating two
geodesic domes. This design produces a regular layout of hexagonal collecting
apertures that optimizes the instrument footprint, aperture diameter,
instrument sensitivity and total field-of-view coverage. We also present the
optical performance of some Fresnel lenses envisaged to develop a dedicated
panoramic SETI (PANOSETI) observatory that will dramatically increase sky-area
searched (pi steradians per dome), wavelength range covered, number of stellar
systems observed, interstellar space examined and duration of time monitored
with respect to previous optical and near-infrared technosignature finders.Comment: 14 pages, 5 figures, 3 table
Geodesic Transport Barriers in Jupiter's Atmosphere: A Video-Based Analysis
Jupiter's zonal jets and Great Red Spot are well known from still images. Yet
the planet's atmosphere is highly unsteady, which suggests that the actual
material transport barriers delineating its main features should be
time-dependent. Rare video footages of Jupiter's clouds provide an opportunity
to verify this expectation from optically reconstructed velocity fields.
Available videos, however, provide short-time and temporally aperiodic velocity
fields that defy classical dynamical systems analyses focused on asymptotic
features. To this end, we use here the recent theory of geodesic transport
barriers to uncover finite-time mixing barriers in the wind field extracted
from a video captured by NASA's Cassini space mission. More broadly, the
approach described here provides a systematic and frame-invariant way to
extract dynamic coherent structures from time-resolved remote observations of
unsteady continua
Light propagation in statistically homogeneous and isotropic universes with general matter content
We derive the relationship of the redshift and the angular diameter distance
to the average expansion rate for universes which are statistically homogeneous
and isotropic and where the distribution evolves slowly, but which have
otherwise arbitrary geometry and matter content. The relevant average expansion
rate is selected by the observable redshift and the assumed symmetry properties
of the spacetime. We show why light deflection and shear remain small. We write
down the evolution equations for the average expansion rate and discuss the
validity of the dust approximation.Comment: 42 pages, no figures. v2: Corrected one detail about the angular
diameter distance and two typos. No change in result
Large sample theory of intrinsic and extrinsic sample means on manifolds--II
This article develops nonparametric inference procedures for estimation and
testing problems for means on manifolds. A central limit theorem for Frechet
sample means is derived leading to an asymptotic distribution theory of
intrinsic sample means on Riemannian manifolds. Central limit theorems are also
obtained for extrinsic sample means w.r.t. an arbitrary embedding of a
differentiable manifold in a Euclidean space. Bootstrap methods particularly
suitable for these problems are presented. Applications are given to
distributions on the sphere S^d (directional spaces), real projective space
RP^{N-1} (axial spaces), complex projective space CP^{k-2} (planar shape
spaces) w.r.t. Veronese-Whitney embeddings and a three-dimensional shape space
\Sigma_3^4.Comment: Published at http://dx.doi.org/10.1214/009053605000000093 in the
Annals of Statistics (http://www.imstat.org/aos/) by the Institute of
Mathematical Statistics (http://www.imstat.org
A graph-based mathematical morphology reader
This survey paper aims at providing a "literary" anthology of mathematical
morphology on graphs. It describes in the English language many ideas stemming
from a large number of different papers, hence providing a unified view of an
active and diverse field of research
- …