1,977 research outputs found
Wavelet-based Faraday Rotation Measure Synthesis
Faraday Rotation Measure (RM) Synthesis, as a method for analyzing
multi-channel observations of polarized radio emission to investigate galactic
magnetic fields structures, requires the definition of complex polarized
intensity in the range of the negative lambda square. We introduce a simple
method for continuation of the observed complex polarized intensity into this
domain using symmetry arguments. The method is suggested in context of magnetic
field recognition in galactic disks where the magnetic field is supposed to
have a maximum in the equatorial plane. The method is quite simple when applied
to a single Faraday-rotating structure on the line of sight. Recognition of
several structures on the same line of sight requires a more sophisticated
technique. We also introduce a wavelet-based algorithm which allows us to
consider a set of isolated structures. The method essentially improves the
possibilities for reconstruction of complicated Faraday structures using the
capabilities of modern radio telescopes.Comment: 5 pages, 5 figures, accepted for publication in MNRAS Letter
Effect of Water Vapor on the Oxidation Behavior of the Eutectic HighâTemperature Alloy Moâ20Siâ52.8Ti
Herein, the effect of water vapor on the oxidation resistance of the alloy Mo-20Si-52.8Ti (at%) is investigated. The alloy is oxidized in dry, wet, as well as in in situ changing atmospheres at 1100 C. The oxidation kinetics changes from nearly parabolic to linear if water vapor is present in oxidizing atmosphere. Under all conditions, the oxide scales consist of an outer TiO and a TiOâSiO duplex layer underneath. In wet atmosphere, the thicknesses of the two regions substantially increase indicating a severe ingress of water vapor. The inferior oxidation resistance in wet environment is primarily rationalized by the fast diffusion of HO through SiO
A View through Faraday's Fog 2: Parsec Scale Rotation Measures in 40 AGN
Results from a survey of the parsec scale Faraday rotation measure properties
for 40 quasars, radio galaxies and BL Lac objects are presented. Core rotation
measures for quasars vary from approximately 500 to several thousand radians
per meter squared. Quasar jets have rotation measures which are typically 500
radians per meter squared or less. The cores and jets of the BL Lac objects
have rotation measures similar to those found in quasar jets. The jets of radio
galaxies exhibit a range of rotation measures from a few hundred radians per
meter squared to almost 10,000 radians per meter squared for the jet of M87.
Radio galaxy cores are generally depolarized, and only one of four radio
galaxies (3C-120) has a detectable rotation measure in the core. Several
potential identities for the foreground Faraday screen are considered and we
believe the most promising candidate for all the AGN types considered is a
screen in close proximity to the jet. This constrains the path length to
approximately 10 parsecs, and magnetic field strengths of approximately 1
microGauss can account for the observed rotation measures. For 27 out of 34
quasars and BL Lacs their optically thick cores have good agreement to a lambda
squared law. This requires the different tau = 1 surfaces to have the same
intrinsic polarization angle independent of frequency and distance from the
black hole.Comment: Accepted to the Astrophysical Journal: 71 pages, 40 figure
A Faraday Rotation Search for Magnetic Fields in Large Scale Structure
Faraday rotation of radio source polarization provides a measure of the
integrated magnetic field along the observational lines of sight. We compare a
new, large sample of Faraday rotation measures (RMs) of polarized extragalactic
sources with galaxy counts in Hercules and Perseus-Pisces, two nearby
superclusters. We find that the average of RMs in these two supercluster areas
are larger than in control areas in the same galactic latitude range. This is
the first RM detection of magnetic fields that pervade a supercluster volume,
in which case the fields are at least partially coherent over several
megaparsecs. Even the most conservative interpretation of our observations,
according to which Milky Way RM variations mimic the background supercluster
galaxy overdensities, puts constraints on the IGM magneto-ionic ``strength'' in
these two superclusters. We obtain an approximate typical upper limit on the
field strength of about 0.3 microGauss l/(500 kpc), when we combine our RM data
with fiducial estimates of electron density from the environments of giant
radio galaxies, and of the warm-hot intergalactic medium (WHIM).Comment: 8 pages, 3 figures, 1 table, to appear in the Astrophysical Journa
Evidence for Ordered Magnetic Fields in the Quasar Environment
At a distance of 20 pc from the purported supermassive black hole powering
quasars, temperatures and densities are inferred from optical observations to
be ~10**4 K and ~10**4 cm**-3. Here we present Very Long Baseline
Interferometry radio observations revealing organized magnetic fields on the
parsec scale in the hot plasma surrounding the quasar OQ172 (1442+101). These
magnetic fields rotate the plane of polarization of the radio emission coming
from the core and inner jet of the quasar. The derived rotation measure (RM) is
40,000 rad m**-2 in the rest frame of the quasar. Only 10 mas (a projected
distance of 68 pc) from the nucleus the jet absolute values of RM fall to less
than 100 rad m**-2.Comment: in press at ApJ Letters, 12 page LaTeX document includes 4 postscript
figure
Ultrafast depolarization of the fluorescence in a conjugated polymer
The effect of the extent of pi electron conjugation on the primary photophysics in semiconducting polymers is reported. A rapid depolarization of photoluminescence and transient absorption, which indicates a reorientation of the transition dipole moment by similar to 30 degrees on a sub-100 fs time scale, is observed in the fully conjugated polymer poly[2-(2'-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEH-PPV). In contrast, partially conjugated polymers exhibit a much slower depolarization. The results reveal rapid changes of exciton delocalization in the fully conjugated MEH-PPV driven by structural relaxation
On right conjugacy closed loops of twice prime order
The right conjugacy closed loops of order 2p, where p is an odd prime, are
classified up to isomorphism.Comment: Clarified definitions, added some remarks and a tabl
Self-Generated Magnetic Fields in Galactic Cooling Flows
Interstellar magnetic fields in elliptical galaxies are assumed to have their
origin in stellar fields that accompany normal mass loss from an evolving
population of old stars. The seed fields are amplified by interstellar
turbulence driven by stellar mass loss and supernova events. These disordered
fields are further amplified by time-dependent compression in the inward moving
galactic cooling flow and are expected to dominate near the galactic core.
Under favorable circumstances, fields similar in strength to those observed G can be generated solely from these natural
galactic processes. In general the interstellar field throughout elliptical
galaxies is determined by the outermost regions in the interstellar gas where
the turbulent dynamo process can occur. Because of the long hydrodynamic flow
times in galactic cooling flows, currently observed magnetic fields may result
from periods of intense turbulent field amplification that occurred in the
outer galaxy in the distant past. Particularly strong fields in ellipticals may
result from ancient galactic mergers or shear turbulence introduced at the
boundary between the interstellar gas and ambient cluster gas.Comment: 21 pages in AASTEX LaTeX with 2 figures; accepted by Astrophysical
Journa
On the reliability of polarization estimation using Rotation Measure Synthesis
We benchmark the reliability of the Rotation Measure (RM) synthesis algorithm
using the 1005 Centaurus A field sources of Feain et al. (2009). The RM
synthesis solutions are compared with estimates of the polarization parameters
using traditional methods. This analysis provides verification of the
reliability of RM synthesis estimates. We show that estimates of the
polarization parameters can be made at lower S/N if the range of RMs is
bounded, but reliable estimates of individual sources with unusual RMs require
unconstrainted solutions and higher S/N.
We derive from first principles the statistical properties of the
polarization amplitude associated with RM synthesis in the presence of noise.
The amplitude distribution depends explicitly on the amplitude of the
underlying (intrinsic) polarization signal. Hence it is necessary to model the
underlying polarization signal distribution in order to estimate the
reliability and errors in polarization parameter estimates. We introduce a
Bayesian method to derive the distribution of intrinsic amplitudes based on the
distribution of measured amplitudes.
The theoretically-derived distribution is compared with the empirical data to
provide quantitative estimates of the probability that an RM synthesis solution
is correct as a function of S/N. We provide quantitative estimates of the
probability that any given RM synthesis solution is correct as a function of
measured polarized amplitude and the intrinsic polarization amplitude compared
to the noise.Comment: accepted for publication in the Astrophysical Journa
Control of mobility in molecular organic semiconductors by dendrimer generation
Conjugated dendrimers are of interest as novel materials for light-emitting diodes. They consist of a luminescent chromophore at the core with highly branched conjugated dendron sidegroups. In these materials, light emission occurs from the core and is independent of generation. The dendron branching controls the separation between the chromophores, We present here a family of conjugated dendrimers and investigate the effect of dendron branching on light emission and charge transport. We apply a number of transport measurement techniques to thin films of a conjugated dendrimer in a light-emitting diode configuration to determine the effect of chromophore spacing on charge transport. We find that the mobility is reduced by two orders of magnitude as the size of the molecule doubles with increased branching or dendrimer generation. The degree of branching allows a unique control of mobility by molecular structure. An increase in chromophore separation also results in a reduction of intermolecular interactions, which reduces the red emission tail in film photoluminescence. We find that the steady-state charge transport is well described by a simple device model incorporating the effect of generation, and use the materials to shed light on the interpretation of transient electroluminescence data. We demonstrate the significance of the ability to tune the mobility in bilayer devices, where a more balanced charge transport can be achieved
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