411 research outputs found
Polarized Emission from Interstellar Dust
Observations of far-infrared (FIR) and submillimeter (SMM) polarized emission
are used to study magnetic fields and dust grains in dense regions of the
interstellar medium (ISM). These observations place constraints on models of
molecular clouds, star-formation, grain alignment mechanisms, and grain size,
shape, and composition. The FIR/SMM polarization is strongly dependent on
wavelength. We have attributed this wavelength dependence to sampling different
grain populations at different temperatures. To date, most observations of
polarized emission have been in the densest regions of the ISM. Extending these
observations to regions of the diffuse ISM, and to microwave frequencies, will
provide additional tests of grain and alignment models.
An understanding of polarized microwave emission from dust is key to an
accurate measurement of the polarization of the cosmic microwave background.
The microwave polarization spectrum will put limits on the contributions to
polarized emission from spinning dust and vibrating magnetic dust.Comment: 19 pages; LaTeX2e uses eas.cls; embeds 10 eps files into 7 Figures in
document. To appear in "Sky Polarisation at Far-infrared to Radio
Wavelengths: The Galactic Screen before the Cosmic Microwave Background,"
eds. M.-A. Miville-Deschenes and F. Boulanger. EAS Publications Series,
Paris. Proceedings of a conference held at IAS, University of Paris-Sud,
September 200
Astronomical Image Processing with Array Detectors
We address the question of astronomical image processing from data obtained
with array detectors. We define and analyze the cases of evenly, regularly, and
irregularly sampled maps for idealized (i.e., infinite) and realistic (i.e.,
finite) detectors. We concentrate on the effect of interpolation on the maps,
and the choice of the kernel used to accomplish this task. We show how the
normalization intrinsic to the interpolation process must be carefully
accounted for when dealing with irregularly sampled grids. We also analyze the
effect of missing or dead pixels in the array, and their consequences for the
Nyquist sampling criterion.Comment: 31 pages, 5 figures, accepted for publication in the PAS
Characterization of Turbulence from Submillimeter Dust Emission
In this paper we use our recent technique for estimating the turbulent
component of the magnetic field to derive the structure functions of the
unpolarized emission as well as that of the Stokes Q and U parameters of the
polarized emission. The solutions for the structure functions to 350-um SHARP
polarization data of OMC-1 allow the determination of the corresponding
turbulent correlation length scales. The estimated values for these length
scales are 9.4" +/- 0.1", 7.3" +/- 0.1", 12.6" +/- 0.2" (or 20.5 +/- 0.2, 16.0
+/- 0.2, and 27.5 +/- 0.4 mpc at 450 pc, the adopted distance for OMC-1) for
the Stokes Q and U parameters, and for the unpolarized emission N,
respectively. Our current results for Q and U are consistent with previous
results obtained through other methods, and may indicate presence of anisotropy
in magnetized turbulence. We infer a weak coupling between the dust component
responsible for the unpolarized emission N and the magnetic field B from the
significant difference between their turbulent correlation length scales.Comment: 14 pages, 3 figures; accepted for publication in the Ap
Placing Confidence Limits on Polarization Measurements
The determination of the true source polarization given a set of measurements
is complicated by the requirement that the polarization always be positive.
This positive bias also hinders construction of upper limits, uncertainties,
and confidence regions, especially at low signal-to-noise levels. We generate
the likelihood function for linear polarization measurements and use it to
create confidence regions and upper limits. This is accomplished by integrating
the likelihood function over the true polarization (parameter space), rather
than the measured polarization (data space). These regions are valid for both
low and high signal-to-noise measurements.Comment: 8 pages, 3 figures, 1 table, submitted to PAS
The Removal of Artificially Generated Polarization in SHARP Maps
We characterize the problem of artificial polarization for the Submillimeter
High Angular Resolution Polarimeter (SHARP) through the use of simulated data
and observations made at the Caltech Submillimeter Observatory (CSO). These
erroneous, artificial polarization signals are introduced into the data through
misalignments in the bolometer sub-arrays plus pointing drifts present during
the data-taking procedure. An algorithm is outlined here to address this
problem and correct for it, provided that one can measure the degree of the
sub-array misalignments and telescope pointing drifts. Tests involving
simulated sources of Gaussian intensity profile indicate that the level of
introduced artificial polarization is highly dependent upon the angular size of
the source. Despite this, the correction algorithm is effective at removing up
to 60% of the artificial polarization during these tests. The analysis of
Jupiter data taken in January 2006 and February 2007 indicates a mean
polarization of 1.44%+/-0.04% and 0.95%+/-0.09%, respectively. The application
of the correction algorithm yields mean reductions in the polarization of
approximately 0.15% and 0.03% for the 2006 and 2007 data sets, respectively.Comment: 19 pages, 7 figure
The effect of signal acquisition and processing choices on ApEn values: Towards a âgold standardâ for distinguishing effort levels from isometric force records
Approximate Entropy (ApEn) is frequently used to identify changes in the complexity of isometric force records with ageing and disease. Different signal acquisition and processing parameters have been used, making comparison or confirmation of results difficult. This study determined the effect of sampling and parameter choices by examining changes in ApEn values across a range of submaximal isometric contractions of the First Dorsal Interosseus. Reducing the sample rate by decimation changed both the value and pattern of ApEn values dramatically. The pattern of ApEn values across the range of effort levels was not sensitive to the filter cut-off frequency, or the criterion used to extract the section of data for analysis. The complexity increased with increasing effort levels using a fixed ârâ value (which accounts for measurement noise) but decreased with increasing effort level when ârâ was set to 0.1 of the standard deviation of force. It is recommended isometric force records are sampled at frequencies >200 Hz, template length (âmâ) is set to 2, and 'r' set to measurement system noise or 0.1 SD depending on physiological process to be distinguished. It is demonstrated that changes in ApEn across effort levels are related to changes in force gradation strategy
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