246 research outputs found
Gilbert Damping in Conducting Ferromagnets II: Model Tests of the Torque-Correlation Formula
We report on a study of Gilbert damping due to particle-hole pair excitations
in conducting ferromagnets. We focus on a toy two-band model and on a four-band
spherical model which provides an approximate description of ferromagnetic
(Ga,Mn)As. These models are sufficiently simple that disorder-ladder-sum vertex
corrections to the long-wavelength spin-spin response function can be summed to
all orders. An important objective of this study is to assess the reliability
of practical approximate expressions which can be combined with electronic
structure calculations to estimate Gilbert damping in more complex systems.Comment: 10 pages, 10 figures. Submitted to Phys. Rev.
Soliton Solutions with Real Poles in the Alekseev formulation of the Inverse-Scattering method
A new approach to the inverse-scattering technique of Alekseev is presented
which permits real-pole soliton solutions of the Ernst equations to be
considered. This is achieved by adopting distinct real poles in the scattering
matrix and its inverse. For the case in which the electromagnetic field
vanishes, some explicit solutions are given using a Minkowski seed metric. The
relation with the corresponding soliton solutions that can be constructed using
the Belinskii-Zakharov inverse-scattering technique is determined.Comment: 8 pages, LaTe
A chaotic long-lived vortex at Venus southern pole
Polar vortices are common in the atmospheres of rapidly rotating planets
[1-4]. On Earth and Mars they are tied to the surface and their existence
follows the seasonal insolation cycle [1-3]. Venus is a slowly rotating planet
but it is also known to have vortices at both poles at the edge of a
superrotating atmosphere [5-8]. However, their nature and long-term properties
have not been constrained so far impeding precise modeling. Here we report
cloud motions at two altitude levels (about 42 km and 63 km above the surface)
using infrared images from the VIRTIS instrument onboard Venus Express that
show that the south polar vortex is a permanent but erratic and unpredictable
feature. We find that the centers of rotation of the vortex at these levels
rarely coincide and both wander erratically around the pole with speeds of up
to 16 m s-1. The cloud morphology and vorticity patches are uncorrelated and
change continuously developing transient areas of small vertical motions. Venus
south polar vortex is a continuously evolving structure immersed in a
baroclinic environment laying at altitude levels that have variable vertical
and meridional wind shears, extending at least 20 km in height through a
quasi-convective turbulent region
Identification of an Extended Accretion Disk Corona in the Hercules X-1 Low State: Moderate Optical Depth, Precise Density Determination, and Verification of CNO Abundances
We identify an accretion disk atmosphere and corona from the high resolution
X-ray spectrum of Hercules X-1, and we determine its detailed physical
properties. More than two dozen recombination emission lines (from Fe XXVI at
1.78 A to N VI at 29.08 A) and Fe K-alpha, K-beta fluorescence lines were
detected in a 50 ks observation with the Chandra High-Energy Transmission
Grating Spectrometer (HETGS). They allow us to measure the density,
temperature, spatial distribution, elemental composition, and kinematics of the
plasma. We exclude HZ Her as the source of the recombination emission. We
compare accretion disk model atmospheres with the observed spectrum in order to
constrain the stratification of density and ionization, disk atmosphere area,
elemental composition, and energetics. The atmospheric spectrum observed during
the low state is photoionized by the main-on X-ray continuum, indicating that
the disk is observed edge-on during the low state. We infer the mean number of
scatterings N of Ly-alpha and Ly-beta line photons from H-like ions. We derive
N < 69 for O VIII Ly_alpha_1, which rules out the presence of a mechanism
modeled by Sako (2003) to enhance N VII emission via a line overlap with O
VIII. The line optical depth diagnostics are consistent with a flattened
atmosphere. Our spectral analysis, the disk atmosphere model, and the presence
of intense N VII and N VI lines (plus N V in the UV), confirm the
over-abundance of nitrogen relative to other metals, which was shown to be
indicative of CNO cycle processing in a massive progenitor.Comment: 38 pages, 14 figures, accepted for publication in Ap
Solution of the 3D-Helmholtz equation in exterior domains using spherical harmonic decomposition
AbstractThis work is devoted to a finite element formulation for the Helmholtz equation in exterior domains. The proposed formulation uses a separation of variables, combining a 2D FE discretization on an intermediate spherical boundary and an ‘a priori’ analytical pattern for the radial direction. Using the analytical radial pattern and the series expansion of trial and test functions in terms of spherical harmonics, an efficient semi-analytical technique is obtained for the direct calculation of the global FE matrices. The accuracy and reliability of the formulation are illustrated through numerical examples of radiation and scattering in the exterior domain
The Hyper Suprime-Cam extended Point Spread Functions and applications to measuring the intra-halo light
We present extended point spread function (PSF) models for the Hyper
Suprime-Cam Subaru Strategic Program Public Data Release 3 (HSC-SSP PDR3) in
all and -bands. Due to its 8.2m primary mirror
and long exposure periods, HSC combines deep images with wide-field coverage,
making it one of the most suitable observing facilities for low surface
brightness (LSB) studies. By applying a median stacking technique of point
sources with different brightnesses, we show how to construct the HSC-SSP PDR3
PSF models to an extent of R 5.6 arcmin. These new PSFs provide the
community with a crucial tool to characterise LSB properties at large angles.
We apply our HSC PSFs and demonstrate that they behave reasonably in two cases:
first, to generate a 2-D model of a bright star, and second, to remove the
PSF-scattered light from an Ultra Deep image of the 400020 Galaxy And Mass
Assembly (GAMA) group in the SXDS field. Our main focus in this second
application is characterising the -band intra-halo light (IHL)
component of 400020. Building on advanced source extraction techniques with
careful consideration of PSF flux, we measure the IHL surface brightness (SB)
group profile up to 31 mag arcsec and R = 300 kpc. We estimate
the IHL fraction () profile, with a mean of
0.13. Our results show that not removing the PSF
light can overestimate the IHL SB by 1.7 mag arcsec and the
by 30%.Comment: 21 pages, 19 figures, submitted to MNRA
Síndrome de encefalopatía posterior reversible en lupus eritematoso sistémico
El síndrome de encefalopatía posterior reversible (PRES) es una condición reversible, poco conocida en el lupus eritematoso sistémico (LES) que puede semejar al lupus neuropsiquiátrico.
Las manifestaciones de PRES son cefalea, convulsiones, alteración del nivel de conciencia y amaurosis. En la mayoría de los casos, la TC (tomografía computarizada) del cerebro muestra lesiones hipodensas en el lóbulo parieto-occipital. Aunque este síndrome es poco común, el reconocimiento rápido y preciso permite un tratamiento temprano con resultados favorables.
Presentamos un caso clínico de una paciente con debut de LES posterior a episodio de eclampsia, y que durante el puerperio tardío presenta hipertensión sostenida asociada a convulsiones y deterioro del nivel de conciencia en contexto de PRES
The Structure and X-ray Recombination Emission of a Centrally Illuminated Accretion Disk Atmosphere and Corona
We model an accretion disk atmosphere and corona photoionized by a central
X-ray continuum source. We calculate the opacity and radiation transfer for an
array of disk radii, to obtain the two-dimensional structure of the disk and
its X-ray recombination emission. The atmospheric structure is insensitive to
the viscosity alpha. We find a feedback mechanism between the disk structure
and the central illumination, which expands the disk and increases the solid
angle subtended by the atmosphere. We model the disk of a neutron star X-ray
binary. We map the temperature, density, and ionization structure of the disk,
and we simulate the high resolution spectra observable with the Chandra and
XMM-Newton grating spectrometers. The X-ray emission lines from the disk
atmosphere are detectable, especially for high-inclination binary systems. The
grating observations of two classes of X-ray binaries already reveal important
spectral similarities with our models. The line spectrum is very sensitive to
the structure of each atmospheric layer, and it probes the heating mechanisms
in the disk. The model spectrum is dominated by double-peaked lines of H-like
and He-like ions, plus weak Fe L. Species with a broad range of ionization
levels coexist at each radius: from Fe XXVI in the hot corona, to C VI at the
base of the atmosphere. The choice of stable solutions affects the spectrum,
since a thermal instability is present in the regime where the X-ray
recombination emission is most intense.Comment: 32 pages, incl. 26 figures, accepted for publication in Ap
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