2,279 research outputs found
Scintillation noise in widefield radio interferometry
In this paper, we consider random phase fluctuations imposed during wave
propagation through a turbulent plasma (e.g. ionosphere) as a source of
additional noise in interferometric visibilities. We derive expressions for
visibility variance for the wide field of view case (FOV deg) by
computing the statistics of Fresnel diffraction from a stochastic plasma, and
provide an intuitive understanding. For typical ionospheric conditions
(diffractive scale km at MHz), we show that the resulting
ionospheric `scintillation noise' can be a dominant source of uncertainty at
low frequencies ( MHz). Consequently, low frequency widefield
radio interferometers must take this source of uncertainty into account in
their sensitivity analysis. We also discuss the spatial, temporal, and spectral
coherence properties of scintillation noise that determine its magnitude in
deep integrations, and influence prospects for its mitigation via calibration
or filtering.Comment: Accepted versio
Femtosecond Demagnetization and Hot Hole Relaxation in Ferromagnetic GaMnAs
We have studied ultrafast photoinduced demagnetization in GaMnAs via
two-color time-resolved magneto-optical Kerr spectroscopy. Below-bandgap
midinfrared pump pulses strongly excite the valence band, while near-infrared
probe pulses reveal sub-picosecond demagnetization that is followed by an
ultrafast (1 ps) partial recovery of the Kerr signal. Through comparison
with InMnAs, we attribute the signal recovery to an ultrafast energy relaxation
of holes. We propose that the dynamical polarization of holes through -
scattering is the source of the observed probe signal. These results support
the physical picture of femtosecond demagnetization proposed earlier for
InMnAs, identifying the critical roles of both energy and spin relaxation of
hot holes.Comment: 7 pages, 6 figure
The Influence of Central Black Holes on Gravitational Lenses
Recent observations indicate that many if not all galaxies host massive
central black holes. In this paper we explore the influence of black holes on
the lensing properties. We model the lens as an isothermal ellipsoid with a
finite core radius plus a central black hole. We show that the presence of the
black hole substantially changes the critical curves and caustics. If the black
hole mass is above a critical value, then it will completely suppress the
central images for all source positions. Realistic central black holes likely
have masses below this critical value. Even in such sub-critical cases, the
black hole can suppress the central image when the source is inside a zone of
influence, which depends on the core radius and black hole mass. In the
sub-critical cases, an additional image may be created by the black hole in
some regions, which for some radio lenses may be detectable with
high-resolution and large dynamic-range VLBI maps. The presence of central
black holes should also be taken into account when one constrains the core
radius from the lack of central images in gravitational lenses.Comment: 7 pages, 3 figures; submitted to MNRA
Spin motive forces due to magnetic vortices and domain walls
We study spin motive forces, i.e, spin-dependent forces, and voltages induced
by time-dependent magnetization textures, for moving magnetic vortices and
domain walls. First, we consider the voltage generated by a one-dimensional
field-driven domain wall. Next, we perform detailed calculations on
field-driven vortex domain walls. We find that the results for the voltage as a
function of magnetic field differ between the one-dimensional and vortex domain
wall. For the experimentally relevant case of a vortex domain wall, the
dependence of voltage on field around Walker breakdown depends qualitatively on
the ratio of the so-called -parameter to the Gilbert damping constant,
and thus provides a way to determine this ratio experimentally. We also
consider vortices on a magnetic disk in the presence of an AC magnetic field.
In this case, the phase difference between field and voltage on the edge is
determined by the parameter, providing another experimental method to
determine this quantity.Comment: 8 pages, 9 figures, submitted to PR
Precession-torque-driven domain-wall motion in out-of-plane materials
Domain-wall (DW) motion in magnetic nanostrips is intensively studied, in
particular because of the possible applications in data storage. In this work,
we will investigate a novel method of DW motion using magnetic field pulses,
with the precession torque as the driving mechanism. We use a one dimensional
(1D) model to show that it is possible to drive DWs in out-of-plane materials
using the precession torque, and we identify the key parameters that influence
this motion. Because the DW moves back to its initial position at the end of
the field pulse, thereby severely complicating direct detection of the DW
motion, depinning experiments are used to indirectly observe the effect of the
precession torque. The 1D model is extended to include an energy landscape in
order to predict the influence of the precession torque in the depinning
experiments. Although preliminary experiments did not yet show an effect of the
precession torque, our calculations indicate that depinning experiments can be
used to demonstrate this novel method of DW motion in out-of-plane materials,
which even allows for coherent motion of multiple domains when the
Dzyaloshinskii-Moriya interaction is taken into account
Gravitational Microlensing Near Caustics I: Folds
We study the local behavior of gravitational lensing near fold catastrophes.
Using a generic form for the lensing map near a fold, we determine the
observable properties of the lensed images, focusing on the case when the
individual images are unresolved, i.e., microlensing. Allowing for images not
associated with the fold, we derive analytic expressions for the photometric
and astrometric behavior near a generic fold caustic. We show how this form
reduces to the more familiar linear caustic, which lenses a nearby source into
two images which have equal magnification, opposite parity, and are equidistant
from the critical curve. In this case, the simplicity and high degree of
symmetry allows for the derivation of semi-analytic expressions for the
photometric and astrometric deviations in the presence of finite sources with
arbitrary surface brightness profiles. We use our results to derive some basic
properties of astrometric microlensing near folds, in particular we predict for
finite sources with uniform and limb darkening profiles, the detailed shape of
the astrometric curve as the source crosses a fold. We find that the
astrometric effects of limb darkening will be difficult to detect with the
currently planned accuracy of the Space Interferometry Mission. We verify our
results by numerically calculating the expected astrometric shift for the
photometrically well-covered Galactic binary lensing event OGLE-1999-BUL-23,
finding excellent agreement with our analytic expressions. Our results can be
applied to any lensing system with fold caustics, including Galactic binary
lenses and quasar microlensing.Comment: 37 pages, 7 figures. Revised version includes an expanded discussion
of applications. Accepted to ApJ, to appear in the August 1, 2002 issue
(v574
SDSS J1650+4251: A New Gravitational Lens
We report that the Sloan Digital Sky Survey quasar SDSS J165043.44+425149.3
is gravitationally lensed into two images, based on observations obtained with
the WIYN 3.5 m telescope at the Kitt Peak National Observatory. The lensed
quasar, at a redshift of z=1.54, appears as two images separated by 1.2" with
B-band magnitudes of 17.8 and 20.0. The lensing galaxy is clearly detected in
I-band images obtained in 0.3" seeing, after point spread function subtraction
of the two quasar images. A strong metal-line absorption system is also
identified in the unresolved SDSS spectrum of the double quasar, suggesting a
plausible lens redshift of z=0.58. The UBRI flux ratios of the pair vary
significantly from 8.5:1 in the blue to 5.4:1 in the red, a difference of 0.5
magnitudes, and may indicate the presence of microlensing in one or both quasar
components. The predicted differential time delay between quasar images is on
the order of one month, assuming the intervening absorption system is due to
the lensing galaxy.Comment: 15 pages (including 4 figures), submitted to the Astronomical Journa
Dynamics of magnetic moments coupled to electrons and lattice oscillations
Inspired by the models of A. Rebei and G. J. Parker and A. Rebei et. al., we
study a physical model which describes the behaviour of magnetic moments in a
ferromagnet. The magnetic moments are associated to 3d electrons which interact
with conduction band electrons and with phonons. We study each interaction
separately and then collect the results assuming that the electron-phonon
interaction can be neglected. For the case of the spin-phonon interaction, we
study the derivation of the equations of motion for the classical spin vector
and find that the correct behaviour, as given by the Brown equation for the
spin vector and the Bloch equation, using the results obtained by D. A. Garanin
for the average over fluctuations of the spin vector, can be obtained in the
high temperature limit. At finite temperatures we show that the Markovian
approximation for the fluctuations is not correct for time scales below some
thermal correlation time . For the case of electrons we workout a
perturbative expansion of the Feynman-Vernon functional. We find the expression
for the random field correlation function. The composite model (as well as the
individual models) is shown to satisfy a fluctuation-dissipation theorem for
all temperature regimes if the behaviour of the coupling constants of the
phonon-spin interaction remains unchanged with the temperature. The equations
of motion are derived
Galaxy Masses
Galaxy masses play a fundamental role in our understanding of structure
formation models. This review addresses the variety and reliability of mass
estimators that pertain to stars, gas, and dark matter. The different sections
on masses from stellar populations, dynamical masses of gas-rich and gas-poor
galaxies, with some attention paid to our Milky Way, and masses from weak and
strong lensing methods, all provide review material on galaxy masses in a
self-consistent manner.Comment: 145 pages, 28 figures, to appear in Reviews of Modern Physics. Figure
22 is missing here, and Figs. 15, 26-28 are at low resolution. This version
has a slightly different title and some typos fixed in Chapter 5. For the
full review with figures, please consult:
http://www.astro.queensu.ca/~courteau/GalaxyMasses_28apr2014.pd
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