381,774 research outputs found
Source Polarization
The notion of source polarization is introduced and investigated. This
complements the earlier work on channel polarization. An application to
Slepian-Wolf coding is also considered. The paper is restricted to the case of
binary alphabets. Extension of results to non-binary alphabets is discussed
briefly.Comment: To be presented at the IEEE 2010 International Symposium on
Information Theory
Compton drag as a mechanism for very high linear polarization in Gamma-Ray Bursts
The recent claim by Coburn & Boggs to have detected a very high degree of
linear polarization in the prompt emission of GRB 021206 has stimulated
interest in how much polarization could arise in gamma-ray bursts from
synchrotron emission. Alternatively, as Shaviv & Dar have shown, GRB
polarization could be produced by inverse Compton scattering in the
point-source limit. We discuss polarization from a fireball that upscatters a
soft radiation field. We show that, after the proper angular integration, the
residual polarization can be large, in some cases approaching the point-source
limit. We discuss the probability of realizing the geometrical conditions in
which a large polarization is obtained showing that, for a particularly bright
burst as GRB 021206, the detection of polarization at the first attempt in the
Compton drag scenario is not unlikely.Comment: Accepted for publication in MNRA
Detection of Circular Polarization in the Galactic Center Black Hole Candidate Sagittarius A*
We report here the detection of circular polarization in the Galactic Center
black hole candidate, Sagittarius A*. The detection was made at 4.8 GHz and 8.4
GHz with the Very Large Array. We find that the fractional circular
polarization at 4.8 GHz is and that the spectral index of
the circular polarization is (). The systematic error in is less than 0.04% at both
frequencies. In light of our recent lower limits on the linear polarization in
Sgr A*, this detection is difficult to interpret with standard models. We
consider briefly whether scattering mechanisms could produce the observed
polarization. Detailed modeling of the source and the scattering medium is
necessary. We propose a simple model in which low energy electrons reduce
linear polarization through Faraday depolarization and convert linear
polarization into circular polarization. Circular polarization may represent a
significant new parameter for studying the obscured centimeter wavelength radio
source in Sgr A*.Comment: ApJL accepted, 11 pages including 1 figur
Ultra-bright source of polarization-entangled photons
Using the process of spontaneous parametric down conversion in a novel
two-crystal geometry, one can generate a source of polarization-entangled
photon pairs which is orders of magnitude brighter than previous sources. We
have measured a high level of entanglement between photons emitted over a
relatively large collection angle, and over a 10-nm bandwidth. As a
demonstration of the source intensity, we obtained a 242- violation of
Bell's inequalities in less than three minutes.Comment: 4 pages, 5 encapsulated Postscript figures. To appear in Physical
Review A (Rapid Communication
Weak-Lensing by Large-Scale Structure and the Polarization Properties of Distant Radio-Sources
We estimate the effects of weak lensing by large-scale density
inhomogeneities and long-wavelength gravitational waves upon the polarization
properties of electromagnetic radiation as it propagates from cosmologically
distant sources. Scalar (density) fluctuations do not rotate neither the plane
of polarization of the electromagnetic radiation nor the source image. They
produce, however, an appreciable shear, which distorts the image shape, leading
to an apparent rotation of the image orientation relative to its plane of
polarization. In sources with large ellipticity the apparent rotation is rather
small, of the order (in radians) of the dimensionless shear. The effect is
larger at smaller source eccentricity. A shear of 1% can induce apparent
rotations of around 5 degrees in radio sources with the smallest eccentricity
among those with a significant degree of integrated linear polarization. We
discuss the possibility that weak lensing by shear with rms value around or
below 5% may be the cause for the dispersion in the direction of integrated
linear polarization of cosmologically distant radio sources away from the
perpendicular to their major axis, as expected from models for their magnetic
fields. A rms shear larger than 5% would be incompatible with the observed
correlation between polarization properties and source orientation in distant
radio galaxies and quasars. Gravity waves do rotate both the plane of
polarization as well as the source image. Their weak lensing effects, however,
are negligible.Comment: 23 pages, 2 eps figures, Aastex 4.0 macros. Final version, as
accepted by ApJ. Additional references and some changes in the introduction
and conclusion
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