123 research outputs found
Physics of Interpulse Emission in Radio Pulsars
The magnetized induced Compton scattering off the particles of the
ultrarelativistic electron-positron plasma of pulsar is considered. The main
attention is paid to the transverse regime of the scattering, which holds in a
moderately strong magnetic field. We specifically examine the problem on
induced transverse scattering of the radio beam into the background, which
takes place in the open field line tube of a pulsar. In this case, the
radiation is predominantly scattered backwards and the scattered component may
grow considerably. Based on this effect, we for the first time suggest a
physical explanation of the interpulse emission observed in the profiles of
some pulsars. Our model can naturally account for the peculiar spectral and
polarization properties of the interpulses. Furthermore, it implies a specific
connection of the interpulse to the main pulse, which may reveal itself in the
consistent intensity fluctuations of the components at different timescales.
Diverse observational manifestations of this connection, including the moding
behavior of PSR B1822-09, the peculiar temporal and frequency structure of the
giant interpulses in the Crab pulsar, and the intrinsic phase correspondence of
the subpulse patterns in the main pulse and the interpulse of PSR B1702-19, are
discussed in detail. It is also argued that the pulse-to-pulse fluctuations of
the scattering efficiency may lead to strong variability of the interpulse,
which is yet to be studied observationally. In particular, some pulsars may
exhibit transient interpulses, i.e. the scattered component may be detectable
only occasionally.Comment: 28 pages, 2 figures. Accepted for publication in Ap
Targeted Feature Recognition Using Mechanical Spatial Filtering with a Low-Cost Compliant Strain Sensor
Partial funding for Open Access provided by the UMD Libraries' Open Access Publishing Fund.A tactile sensing architecture is presented for detection of surface features that have a particular target size, and the concept is demonstrated with a braille pattern. The approach is akin to an inverse of mechanical profilometry. The sensing structure is constructed by suspending a stretchable strainsensing membrane over a cavity. The structure is moved over the surface, and a signal is generated through mechanical spatial filtering if a feature is small enough to penetrate into the cavity. This simple design is tailorable and can be realized by standard machining or 3D printing. Images of target features can be produced with even a low-cost compliant sensor. In this work a disposable elastomeric piezoresistive strain sensor was used over a cylindrical “finger” part with a groove having a width corresponding to the braille dot size. A model was developed to help understand the working principle and guide finger design, revealing amplification when the cavity matches the feature size. The new sensing concept has the advantages of being easily reconfigured for a variety of sensing problems and retrofitted to a wide range of robotic hands, as well as compatibility with many compliant sensor types
Phaseless VLBI mapping of compact extragalactic radio sources
The problem of phaseless aperture synthesis is of current interest in
phase-unstable VLBI with a small number of elements when either the use of
closure phases is not possible (a two-element interferometer) or their quality
and number are not enough for acceptable image reconstruction by standard
adaptive calibration methods. Therefore, we discuss the problem of unique image
reconstruction only from the spectrum magnitude of a source. We suggest an
efficient method for phaseless VLBI mapping of compact extragalactic radio
sources. This method is based on the reconstruction of the spectrum magnitude
for a source on the entire UV plane from the measured visibility magnitude on a
limited set of points and the reconstruction of the sought-for image of the
source by Fienup's method from the spectrum magnitude reconstructed at the
first stage. We present the results of our mapping of the extragalactic radio
source 2200 +420 using astrometric and geodetic observations on a global VLBI
array. Particular attention is given to studying the capabilities of a
two-element interferometer in connection with the putting into operation of a
Russian-made radio interferometer based on Quasar RT-32 radio telescopes.Comment: 21 pages, 6 figure
A Survey of Finite Algebraic Geometrical Structures Underlying Mutually Unbiased Quantum Measurements
The basic methods of constructing the sets of mutually unbiased bases in the
Hilbert space of an arbitrary finite dimension are discussed and an emerging
link between them is outlined. It is shown that these methods employ a wide
range of important mathematical concepts like, e.g., Fourier transforms, Galois
fields and rings, finite and related projective geometries, and entanglement,
to mention a few. Some applications of the theory to quantum information tasks
are also mentioned.Comment: 20 pages, 1 figure to appear in Foundations of Physics, Nov. 2006 two
more references adde
Anisotropy in Bianchi-type brane cosmologies
The behavior near the initial singular state of the anisotropy parameter of
the arbitrary type, homogeneous and anisotropic Bianchi models is considered in
the framework of the brane world cosmological models. The matter content on the
brane is assumed to be an isotropic perfect cosmological fluid, obeying a
barotropic equation of state. To obtain the value of the anisotropy parameter
at an arbitrary moment an evolution equation is derived, describing the
dynamics of the anisotropy as a function of the volume scale factor of the
Universe. The general solution of this equation can be obtained in an exact
analytical form for the Bianchi I and V types and in a closed form for all
other homogeneous and anisotropic geometries. The study of the values of the
anisotropy in the limit of small times shows that for all Bianchi type
space-times filled with a non-zero pressure cosmological fluid, obeying a
linear barotropic equation of state, the initial singular state on the brane is
isotropic. This result is obtained by assuming that in the limit of small times
the asymptotic behavior of the scale factors is of Kasner-type. For brane
worlds filled with dust, the initial values of the anisotropy coincide in both
brane world and standard four-dimensional general relativistic cosmologies.Comment: 12 pages, no figures, to appear in Class. Quantum Gra
A Solution to the Problem of Phaseless Mapping for a High-Orbit Space-Ground Radio Interferometer
We consider the problem of mapping with ultra-high angular resolution using a
space-ground radio interferometer with a space antenna in a high orbit,whose
apogee height exceeds the radius of the Earth by a factor of ten. In this case,
a multielement interferometer essentially degenerates into a two-element
interferometer. The degeneracy of the close-phase relations prevents the use of
standard methods for hybrid mapping and self-calibration for the correct
reconstruction of images. We propose a new phaseless mapping method based on
methods for the reconstruction of images in the complete absence of phase
information, using only the amplitudes of the spatial-coherence function of the
source. In connection with this problem, we propose a new method for the
reliable solution of the phase problem, based on optimizing
information-carrying nonlinear functionals, in particular, the Shannon entropy.
Results of simulations of mapping radio sources with various structures with
ultra-high angular resolution in the framework of the RADIOASTRON mission are
presented.Comment: 15 pages, 7 figure
Wavefront shaping with disorder-engineered metasurfaces
Recently, wavefront shaping with disordered media has demonstrated optical manipulation capabilities beyond those of conventional optics, including extended volume, aberration-free focusing and subwavelength focusing. However, translating these capabilities to useful applications has remained challenging as the input–output characteristics of the disordered media (P variables) need to be exhaustively determined via O(P) measurements. Here, we propose a paradigm shift where the disorder is specifically designed so its exact input–output characteristics are known a priori and can be used with only a few alignment steps. We implement this concept with a disorder-engineered metasurface, which exhibits additional unique features for wavefront shaping such as a large optical memory effect range in combination with a wide angular scattering range, excellent stability, and a tailorable angular scattering profile. Using this designed metasurface with wavefront shaping, we demonstrate high numerical aperture (NA > 0.5) focusing and fluorescence imaging with an estimated ~2.2 × 10^8 addressable points in an ~8 mm field of view
Functional EPAS1/ HIF2A Missense Variant Is Associated With Hematocrit in Andean Highlanders
Hypoxia-inducible factor pathway genes are linked to adaptation in both human and nonhuman highland species. EPAS1, a notable target of hypoxia adaptation, is associated with relatively lower hemoglobin concentration in Tibetans. We provide evidence for an association between an adaptive EPAS1 variant (rs570553380) and the same phenotype of relatively low hematocrit in Andean highlanders. This Andean-specific missense variant is present at a modest frequency in Andeans and absent in other human populations and vertebrate species except the coelacanth. CRISPR-base-edited human cells with this variant exhibit shifts in hypoxia-regulated gene expression, while metabolomic analyses reveal both genotype and phenotype associations and validation in a lowland population. Although this genocopy of relatively lower hematocrit in Andean highlanders parallels well-replicated findings in Tibetans, it likely involves distinct pathway responses based on a protein-coding versus noncoding variants, respectively. These findings illuminate how unique variants at EPAS1 contribute to the same phenotype in Tibetans and a subset of Andean highlanders despite distinct evolutionary trajectories
The Dynamics of Brane-World Cosmological Models
Brane-world cosmology is motivated by recent developments in string/M-theory
and offers a new perspective on the hierarchy problem. In the brane-world
scenario, our Universe is a four-dimensional subspace or {\em brane} embedded
in a higher-dimensional {\em bulk} spacetime. Ordinary matter fields are
confined to the brane while the gravitational field can also propagate in the
bulk, leading to modifications of Einstein's theory of general relativity at
high energies. In particular, the Randall-Sundrum-type models are
self-consistent and simple and allow for an investigation of the essential
non-linear gravitational dynamics. The governing field equations induced on the
brane differ from the general relativistic equations in that there are nonlocal
effects from the free gravitational field in the bulk, transmitted via the
projection of the bulk Weyl tensor, and the local quadratic energy-momentum
corrections, which are significant in the high-energy regime close to the
initial singularity. In this review we discuss the asymptotic dynamical
evolution of spatially homogeneous brane-world cosmological models containing
both a perfect fluid and a scalar field close to the initial singularity. Using
dynamical systems techniques it is found that, for models with a physically
relevant equation of state, an isotropic singularity is a past-attractor in all
orthogonal spatially homogeneous models (including Bianchi type IX models). In
addition, we describe the dynamics in a class of inhomogeneous brane-world
models, and show that these models also have an isotropic initial singularity.
These results provide support for the conjecture that typically the initial
cosmological singularity is isotropic in brane-world cosmology.Comment: Einstein Centennial Review Article: to appear in CJ
Feasibility studies for the measurement of time-like proton electromagnetic form factors from p¯ p→ μ+μ- at P ¯ ANDA at FAIR
This paper reports on Monte Carlo simulation results for future measurements of the moduli of time-like proton electromagnetic form factors, | GE| and | GM| , using the p¯ p→ μ+μ- reaction at P ¯ ANDA (FAIR). The electromagnetic form factors are fundamental quantities parameterizing the electric and magnetic structure of hadrons. This work estimates the statistical and total accuracy with which the form factors can be measured at P ¯ ANDA , using an analysis of simulated data within the PandaRoot software framework. The most crucial background channel is p¯ p→ π+π-, due to the very similar behavior of muons and pions in the detector. The suppression factors are evaluated for this and all other relevant background channels at different values of antiproton beam momentum. The signal/background separation is based on a multivariate analysis, using the Boosted Decision Trees method. An expected background subtraction is included in this study, based on realistic angular distributions of the background contribution. Systematic uncertainties are considered and the relative total uncertainties of the form factor measurements are presented
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