212 research outputs found
Microarcsecond Radio Imaging using Earth Orbit Synthesis
The observed interstellar scintillation pattern of an intra-day variable
radio source is influenced by its source structure. If the velocity of the
interstellar medium responsible for the scattering is comparable to the
earth's, the vector sum of these allows an observer to probe the scintillation
pattern of a source in two dimensions and, in turn, to probe two-dimensional
source structure on scales comparable to the angular scale of the scintillation
pattern, typically as for weak scattering. We review the theory on
the extraction of an ``image'' from the scintillation properties of a source,
and show how earth's orbital motion changes a source's observed scintillation
properties during the course of a year. The imaging process, which we call
Earth Orbit Synthesis, requires measurements of the statistical properties of
the scintillations at epochs spread throughout the course of a year.Comment: ApJ in press. 25 pages, 7 fig
Primary Beam Shape Calibration from Mosaicked, Interferometric Observations
Image quality in mosaicked observations from interferometric radio telescopes
is strongly dependent on the accuracy with which the antenna primary beam is
calibrated. The next generation of radio telescope arrays such as the Allen
Telescope Array (ATA) and the Square Kilometer Array (SKA) have key science
goals that involve making large mosaicked observations filled with bright point
sources. We present a new method for calibrating the shape of the telescope's
mean primary beam that uses the multiple redundant observations of these bright
sources in the mosaic. The method has an analytical solution for simple
Gaussian beam shapes but can also be applied to more complex beam shapes
through minimization. One major benefit of this simple, conceptually
clean method is that it makes use of the science data for calibration purposes,
thus saving telescope time and improving accuracy through simultaneous
calibration and observation. We apply the method both to 1.43 GHz data taken
during the ATA Twenty Centimeter Survey (ATATS) and to 3.14 GHz data taken
during the ATA's Pi Gigahertz Sky Survey (PiGSS). We find that the beam's
calculated full width at half maximum (FWHM) values are consistent with the
theoretical values, the values measured by several independent methods, and the
values from the simulation we use to demonstrate the effectiveness of our
method on data from future telescopes such as the expanded ATA and the SKA.
These results are preliminary, and can be expanded upon by fitting more complex
beam shapes. We also investigate, by way of a simulation, the dependence of the
accuracy of the telescope's FWHM on antenna number. We find that the
uncertainty returned by our fitting method is inversely proportional to the
number of antennas in the array.Comment: Accepted by PASP. 8 pages, 8 figure
The Precision Array for Probing the Epoch of Reionization: 8 Station Results
We are developing the Precision Array for Probing the Epoch of Reionization
(PAPER) to detect 21cm emission from the early Universe, when the first stars
and galaxies were forming. We describe the overall experiment strategy and
architecture and summarize two PAPER deployments: a 4-antenna array in the
low-RFI environment of Western Australia and an 8-antenna array at our
prototyping site in Green Bank, WV. From these activities we report on system
performance, including primary beam model verification, dependence of system
gain on ambient temperature, measurements of receiver and overall system
temperatures, and characterization of the RFI environment at each deployment
site.
We present an all-sky map synthesized between 139 MHz and 174 MHz using data
from both arrays that reaches down to 80 mJy (4.9 K, for a beam size of 2.15e-5
steradians at 154 MHz), with a 10 mJy (620 mK) thermal noise level that
indicates what would be achievable with better foreground subtraction. We
calculate angular power spectra () in a cold patch and determine them
to be dominated by point sources, but with contributions from galactic
synchrotron emission at lower radio frequencies and angular wavemodes. Although
the cosmic variance of foregrounds dominates errors in these power spectra, we
measure a thermal noise level of 310 mK at for a 1.46-MHz band
centered at 164.5 MHz. This sensitivity level is approximately three orders of
magnitude in temperature above the level of the fluctuations in 21cm emission
associated with reionization.Comment: 13 pages, 14 figures, submitted to AJ. Revision 2 corrects a scaling
error in the x axis of Fig. 12 that lowers the calculated power spectrum
temperatur
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Neoliberal Redistributive Policy: The U.S. Net Social Wage in the 21st Century
In this paper, I examine the trends of fiscal transfers between the state and workers during 1959 - 2012 to understand the net impact of redistributive policy in the United States. This paper presents original net social wage data from and analysis based on the replication and extension of Shaikh and Tonak (2002). The paper investigates the appearance of a post-2001 variation in the net social wage data. The positive net social wage in the 21st century is the result of a combination of factors including the growth of income support, healthcare inflation, neoliberal tax reforms, and macroeconomic instability. Growing economic inequality does not appear to alter the results of the net social wage methodology
Departures From Axisymmetric Morphology and Dynamics in Spiral Galaxies
New HI synthesis data have been obtained for six face-on galaxies with the
Very Large Array. These data and reanalyses of three additional data sets make
up a sample of nine face-on galaxies analyzed for deviations from axisymmetry
in morphology and dynamics. This sample represents a subsample of galaxies
already analyzed for morphological symmetry properties in the R-band. Four
quantitative measures of dynamical nonaxisymmetry are compared to one another
and to the quantitative measures of morphological asymmetry in HI and R-band to
investigate the relationships between nonaxisymmetric morphology and dynamics.
We find no significant relationship between asymmetric morphology and most of
the dynamical measures in our sample. A possible relationship is found,
however, between morphology and dynamical position angle differences between
approaching and receding sides of the galaxy.Comment: 24 pages, 19 figures, AASTeX, accepted for publication in AJ,
postscript figures available at
ftp://culebra.tn.cornell.edu/pub/david/figures.tar.g
A Per-Baseline, Delay-Spectrum Technique for Accessing the 21cm Cosmic Reionization Signature
A critical challenge in measuring the power spectrum of 21cm emission from
cosmic reionization is compensating for the frequency dependence of an
interferometer's sampling pattern, which can cause smooth-spectrum foregrounds
to appear unsmooth and degrade the separation between foregrounds and the
target signal. In this paper, we present an approach to foreground removal that
explicitly accounts for this frequency dependence. We apply the delay
transformation introduced in Parsons & Backer (2009) to each baseline of an
interferometer to concentrate smooth-spectrum foregrounds within the bounds of
the maximum geometric delays physically realizable on that baseline. By
focusing on delay-modes that correspond to image-domain regions beyond the
horizon, we show that it is possible to avoid the bulk of smooth-spectrum
foregrounds. We map the point-spread function of delay-modes to k-space,
showing that delay-modes that are uncorrupted by foregrounds also represent
samples of the three-dimensional power spectrum, and can be used to constrain
cosmic reionization. Because it uses only spectral smoothness to differentiate
foregrounds from the targeted 21cm signature, this per-baseline analysis
approach relies on spectrally- and spatially-smooth instrumental responses for
foreground removal. For sufficient levels of instrumental smoothness relative
to interfering foregrounds, this technique substantially reduces the level of
calibration previously thought necessary to detect 21cm reionization. As a
result, this approach places fewer constraints on antenna configuration within
an array, facilitating the adoption of configurations optimized for
power-spectrum sensitivity. Under these assumptions, we demonstrate the
potential for PAPER to detect 21cm reionization at an amplitude of 10 mK^2 near
k~0.2h Mpc^-1 with 132 dipoles in 7 months of observing.Comment: 33 Pages, 11 figures, accepted to Ap
The Merits of Universal Scholarships: Benefit-Cost Evidence from the Kalamazoo Promise
Sequential compression devices in postoperative urologic patients: an observational trial and survey study on the influence of patient and hospital factors on compliance
Implementation and engagement of the SMART Work & Life sitting reduction intervention: an exploratory analysis on intervention effectiveness
Background: To enhance the impact of interventions, it is important to understand how intervention engagement relates to study outcomes. We report on the level of implementation and engagement with the SMART Work & Life (SWAL) programme (delivered with (SWAL plus desk) and without a height-adjustable desk (SWAL)) and explore the effects of different levels of this on change in daily sitting time in comparison to the control group. Methods: The extent of intervention delivery by workplace champions and the extent of engagement by champions and participants (staff) with each intervention activity was assessed by training attendance logs, workplace champion withdrawal dates, intervention activities logs and questionnaires. These data were used to assess whether a cluster met defined criteria for low, medium, or high implementation and engagement or none of these. Mixed effects linear regression analyses tested whether change in sitting time varied by: (i) the number of intervention activities implemented and engaged with, and (ii) the percentage of implementation and engagement with all intervention strategies. Results: Workplace champions were recruited for all clusters, with 51/52 (98%) attending training. Overall, 12/27 (44.4%) SWAL and 9/25 (36.0%) SWAL plus desk clusters implemented all main intervention strategies. Across remaining clusters, the level of intervention implementation varied. Those in the SWAL (n = 8 (29.6%) clusters, 80 (32.1%) participants) and SWAL plus desk (n = 5 (20.0%) clusters, 41 (17.1%) participants) intervention groups who implemented and engaged with the most intervention strategies and had the highest percentage of cluster implementation and engagement with all intervention strategies sat for 30.9 (95% CI -53.9 to -7.9, p = 0.01) and 75.6 (95% CI -103.6 to -47.7, p < 0.001) fewer minutes/day respectively compared to the control group at 12 month follow up. These differences were larger than the complete case analysis. The differences in sitting time observed for the medium and low levels were similar to the complete case analysis. Conclusions: Most intervention strategies were delivered to some extent across the clusters although there was large variation. Superior effects for sitting reduction were seen for those intervention groups who implemented and engaged with the most intervention components and had the highest level of cluster implementation and engagement. Trial Registration: ISRCTN11618007. Registered on 24 January 2018. https://www.isrctn.com/ISRCTNISRCTN11618007
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