163 research outputs found
On Measuring Accurate 21-cm Line Profiles with the Robert C. Byrd Green Bank Telescope
We use observational data to show that 21 cm line profiles measured with the
Green Bank Telescope (GBT) are subject to significant inaccuracy. These include
~10% errors in the calibrated gain and significant contribution from distant
sidelobes. In addition, there are ~60% variations between the GBT and
Leiden/Argentine/Bonn 21 cm line profile intensities, which probably occur
because of the high main-beam efficiency of the GBT. Stokes V profiles from the
GBT contain inaccuracies that are related to the distant sidelobes.
We illustrate these problems, define physically motivated components for the
sidelobes, and provide numerical results showing the inaccuracies. We provide a
correction scheme for Stokes I 21 cm line profiles that is fairly successful
and provide some rule-of-thumb comments concerning the accuracy of Stokes V
profiles.Comment: 39 pages, 20 figures, accepted for publication in PAS
A New Technique for Heterodyne Spectroscopy: Least-Squares Frequency Switching (LSFS)
We describe a new technique for heterodyne spectroscopy, which we call
Least-Squares Frequency Switching, or LSFS. This technique avoids the need for
a traditional reference spectrum, which--when combined with the on-source
spectrum--introduces both noise and systematic artifacts such as ``baseline
wiggles''. In contrast, LSFS derives the spectrum directly, and in addition the
instrumental gain profile. The resulting spectrum retains nearly the full
theoretical sensitivity and introduces no systematic artifacts.
Here we discuss mathematical details of the technique and use numerical
experiments to explore optimum observing schemas. We outline a modification
suitable for computationally difficult cases as the number of spectral channels
grows beyond several thousand. We illustrate the method with three real-life
examples. In one of practical interest, we created a large contiguous bandwidth
aligning three smaller bandwidths end-to-end; radio astronomers are often faced
with the need for a larger contiguous bandwidth than is provided with the
available correlator.Comment: 37 pages, 8 figure
Constraints on OH Megamaser Excitation from a Survey of OH Satellite Lines
We report the results of a full-Stokes survey of all four 18 cm OH lines in
77 OH megamasers (OHMs) using the Arecibo Observatory. This is the first survey
of OHMs that included observations of the OH satellite lines; only 4 of the 77
OHMs have existing satellite line observations in the literature. In 5 sources,
satellite line emission is detected, with 3 of the 5 sources re-detections of
previously published sources. The 2 sources with new detections of satellite
line emission are IRAS F10173+0829, which was detected at 1720 MHz, and IRAS
F15107+0724, for which both the 1612 MHz and 1720 MHz lines were detected. In
IRAS F15107+0724, the satellite lines are partially conjugate, as 1720 MHz
absorption and 1612 MHz emission have the same structure at some velocities
within the source, along with additional broader 1612 MHz emission. This is the
first observed example of conjugate satellite lines in an OHM. In the remaining
sources, no satellite line emission is observed. The detections and upper
limits are generally consistent with models of OHM emission in which all of the
18 cm OH lines have the same excitation temperature. There is no evidence for a
significant population of strong satellite line emitters among OHMs.Comment: 9 pages, accepted to Ap
Our Local Microcosmos
UM Department of Astronomy, UM Student Astronomical Society, UM Exhibit Museum of Natural History, Michigan Center for Theoretical Physicshttp://deepblue.lib.umich.edu/bitstream/2027.42/57277/3/our_local_microcosmos.MP3http://deepblue.lib.umich.edu/bitstream/2027.42/57277/2/our_local_microcosmos.pd
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