173 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
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
Origins of the 1/4 keV Soft X-Ray Background
Snowden and coworkers have presented a model for the 1/4 keV soft X-ray
diffuse background in which the observed flux is dominated by a ~ 10^6 K
thermal plasma located in a 100-300 pc diameter bubble surrounding the Sun, but
has significant contributions from a very patchy Galactic halo. Halo emission
provides about 11% of the total observed flux and is responsible for half of
the H I anticorrelation. The remainder of the anticorrelation is presumably
produced by displacement of disk H I by the varying extent of the local hot
bubble (LHB). The ROSAT R1 and R2 bands used for this work had the unique
spatial resolution and statistical precision required for separating the halo
and local components, but provide little spectral information. Some consistency
checks had been made with older observations at lower X-ray energies, but we
have made a careful investigation of the extent to which the model is supported
by existing sounding rocket data in the Be (73-111 eV) and B bands (115-188 eV)
where the sensitivities to the model are qualitatively different from the ROSAT
bands. We conclude that the two-component model is well supported by the
low-energy data. We find that these combined observations of the local
component may be consistent with single-temperature thermal emission models in
collisional ionization equilibrium if depleted abundances are assumed. However,
different model implementations give significantly different results, offering
little support for the conclusion that the astrophysical situation is so
simple.Comment: 17 pages, 6 figures, accepted by the Astrophysical Journa
The Submillimeter Polarization Spectrum of M17
We present 450 {\mu}m polarimetric observations of the M17 molecular cloud
obtained with the SHARP polarimeter at the Caltech Submillimeter Observatory.
Across the observed region, the magnetic field orientation is consistent with
previous submillimeter and far-infrared polarization measurements. Our
observations are centered on a region of the molecular cloud that has been
compressed by stellar winds from a cluster of OB stars. We have compared these
new data with previous 350 {\mu}m polarimetry and find an anti-correlation
between the 450 to 350 {\mu}m polarization magnitude ratio and the ratio of 21
cm to 450 {\mu}m intensity. The polarization ratio is lower near the east end
of the studied region where the cloud is exposed to stellar winds and
radiation. At the west end of the region, the polarization ratio is higher. We
interpret the varying polarization spectrum as evidence supporting the
radiative alignment torque (RAT) model for grain alignment, implying higher
alignment efficiency in the region that is exposed to a higher anisotropic
radiation field.Comment: 24 pages, 10 figure
Dispersion of Magnetic Fields in Molecular Clouds. II.
We expand our study on the dispersion of polarization angles in molecular clouds. We show how the effect of signal integration through the thickness of the cloud as well as across the area subtended by the telescope beam inherent to dust continuum measurements can be incorporated in our analysis to correctly account for its effect on the measured angular dispersion and inferred turbulent to large-scale magnetic field strength ratio. We further show how to evaluate the turbulent magnetic field correlation scale from polarization data of sufficient spatial resolution and high enough spatial sampling rate. We apply our results to the molecular cloud OMC-1, where we find a turbulent correlation length of ÎŽ â 16 mpc, a turbulent to large-scale magnetic field strength ratio of approximately 0.5, and a plane-of-the-sky large-scale magnetic field strength of approximately 760 ÎŒG
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