128 research outputs found
A Near-Field Treatment of Aperture Synthesis Techniques using the Murchison Widefield Array
Typical radio interferometer observations are performed assuming the source
of radiation to be in the far-field of the instrument, resulting in a
two-dimensional Fourier relationship between the observed visibilities in the
aperture plane and the sky brightness distribution (over a small field of
view). When near-field objects are present in an observation, the standard
approach applies far-field delays during correlation, resulting in loss of
signal coherence for the signal from the near-field object. In this paper, we
demonstrate near-field aperture synthesis techniques using a Murchison
Widefield Array observation of the International Space Station (ISS), as it
appears as a bright near-field object. We perform visibility phase corrections
to restore coherence across the array for the near-field object (however not
restoring coherence losses due to time and frequency averaging at the
correlator). We illustrate the impact of the near-field corrections in the
aperture plane and the sky plane. The aperture plane curves to match the
curvature of the near-field wavefront, and in the sky plane near-field
corrections manifest as fringe rotations at different rates as we bring the
focal point of the array from infinity to the desired near-field distance. We
also demonstrate the inverse scenario of inferring the line-of-sight range of
the ISS by inverting the apparent curvature of the wavefront seen by the
aperture. We conclude the paper by briefly discussing the limitations of the
methods developed and the near-field science cases where our approach can be
exploited.Comment: Accepted in Publications of the Astronomical Society of Australia
(PASA). 10 pages, 7 figures, and lots of linked animation
Ten milliparsec-scale structure of the nucleus region in Centaurus A
We present the results of a VLBI Space Observatory Programme (VSOP)
observation of the subparsec structure in Centaurus A at 4.9 GHz. Owing to its
proximity, our Centaurus A space-VLBI image is one of the highest spatial
resolution images of an AGN ever made -- 0.01 pc per beam. The elongated core
region is resolved into several components over 10 milli-arcseconds long (0.2
pc) including a compact component of brightness temperature 2.2x10^10K. We
analyze the jet geometry in terms of collimation. Assuming the strongest
component to be the core, the jet opening angle at ~ 5,000 r_s (Schwarzchild
radii) from the core is estimated to be ~ 12 degree, with collimation of the
jet to ~ 3 degree continuing out to ~ 20,000 r_s. This result is consistent
with previous studies of the jet in M87, which favor MHD disk outflow models.
Future space VLBI observations at higher frequencies will probably be able to
image the collimation region, within 1,000 r_s of the center of Centaurus A,
together with the accretion disk itself.Comment: 12 pages, 6 figures, accepted for publication in PASJ, Vol.57 No.6,
VSOP special issu
e-VLBI observations of GHz-Peaked Spectrum (GPS) radio sources in nearby galaxies from the AT20G survey
GHz-peaked spectrum (GPS) radio sources are thought to be young objects which
later evolve into FR-I and FR-II radio galaxies. We have used the Australia
Telescope 20GHz (AT20G) survey catalogue to select a uniform sample of GPS
sources with spectral peaks above 5GHz, which should represent the youngest
members of this class. In this paper, we present e-VLBI observations of ten
such objects which are associated with nearby (z<0.15) galaxies and so
represent a new population of local, low--power GPS sources. Our e-VLBI
observations were carried out at 4.8GHz with the Australia Telescope Long
Baseline Array (LBA) using a real--time software correlator. All ten sources
were detected, and were unresolved on scales of ~100mas, implying that they are
typically less than 100pc in linear size.Comment: 7 pages, 7 figures, 3 table
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