295 research outputs found
The Life and Times of the Parkes-Tidbinbilla Interferometer
The Parkes-Tidbinbilla took advantage of a real-time radio-link connecting
the Parkes and Tidbinbilla antennas to form the world's longest real-time
interferometer. Built on a minuscule budget, it was an extraordinarily
successful instrument, generating some 24 journal papers including 3 Nature
papers, as well as facilitating the early development of the Australia
Telescope Compact Array. Here we describe its origins, construction, successes,
and life cycle, and discuss the future use of single-baseline interferometers
in the era of SKA and its pathfinders.Comment: Accepted by Journal of Astronomical History & Heritage. arXiv admin
note: substantial text overlap with arXiv:1210.098
Adaptive Filters Revisited - RFI Mitigation in pulsar observations
Pulsar detection and timing experiments are applications where adaptive
filters seem eminently suitable tools for radio-frequency-interference (RFI)
mitigation. We describe a novel variant which works well in field trials of
pulsar observations centred on an observing frequency of 675 MHz, a bandwidth
of 64 MHz and with 2-bit sampling. Adaptive filters have generally received bad
press for RFI mitigation in radio astronomical observations with their most
serious drawback being a spectral echo of the RFI embedded in the filtered
signals. Pulsar observations are intrinsically less sensitive to this as they
operate in the (pulsar period) time domain. The field trials have allowed us to
identify those issues which limit the effectiveness of the adaptive filter. We
conclude that adaptive filters can significantly improve pulsar observations in
the presence of RFI.Comment: Accepted for publication in Radio Scienc
An Australia telescope survey for CMB anisotropies
We have surveyed six distinct `empty fields' using the Australia Telescope
Compact Array in an ultra-compact configuration with the aim of imaging, with a
high brightness sensitivity, any arcmin-scale brightness-temperature
anisotropies in the background radio sky. The six well-separated regions were
observed at a frequency of 8.7 GHz and the survey regions were limited by the
ATCA primary beams which have a full width at half maximum of 6 arcmin at this
frequency; all fields were observed with a resolution of 2 arcmin and an rms
thermal noise of 24 microJy/beam. After subtracting foreground confusion
detected in higher resolution images of the fields, residual fluctuations in
Stokes I images are consistent with the expectations from thermal noise and
weaker (unidentified) foreground sources; the Stokes Q and U images are
consistent with expectations from thermal noise.
Within the sensitivity of our observations, we have no reason to believe that
there are any Sunyaev-Zeldovich holes in the microwave sky surveyed. Assuming
Gaussian-form CMB anisotropy with a `flat' spectrum, we derive 95 per cent
confidence upper limits of Q_flat < 10--11 microK in polarized intensity and
Q_flat < 25 microK in total intensity. The ATCA filter function peaks at l=4700
and has half maximum values at l=3350 and 6050.Comment: 17 pages, includes 8 figures and 6 tables, accepted for publication
in MNRA
Compact Radio Cores in Seyfert Galaxies
We have observed a sample of 157 Seyfert galaxies with a 275 km baseline
radio interferometer to search for compact, high brightness temperature radio
emission from the active nucleus. We obtain the surprising result that compact
radio cores are much more common in Seyfert 2 than in Seyfert 1 galaxies, which
at first seems to be inconsistent with orientation unification schemes. We
propose a model, involving optical depth effects in the narrow-line region,
which can reconcile our result with the standard unified scheme. (Accepted for
publication in ApJ 1994 Sep 10)Comment: 21 pages and 7 figures, uuencoded tar-compressed postscript files,
ATP18
Deep Chandra Observation of the Pulsar Wind Nebula Powered by the Pulsar J1846-0258 in the Supernova Remnant Kes 75
We present the results of detailed spatial and spectral analysis of the
pulsar wind nebula (PWN) in supernova remnant Kes 75 (G29.7-0.3) using a deep
exposure with Chandra X-ray observatory. The PWN shows a complex morphology
with clear axisymmetric structure. We identified a one-sided jet and two bright
clumps aligned with the overall nebular elongation, and an arc-like feature
perpendicular to the jet direction. Further spatial modeling with a torus and
jet model indicates a position angle 207\arcdeg\pm8 \arcdeg for the PWN
symmetry axis. We interpret the arc as an equatorial torus or wisp and the
clumps could be shock interaction between the jets and the surrounding medium.
The lack of any observable counter jet implies a flow velocity larger than
0.4c. Comparing to an archival observation 6 years earlier, some small-scale
features in the PWN demonstrate strong variability: the flux of the inner jet
doubles and the peak of the northern clump broadens and shifts 2" outward. In
addition, the pulsar flux increases by 6 times, showing substantial spectral
softening from =1.1 to 1.9 and an emerging thermal component which was
not observed in the first epoch. The changes in the pulsar spectrum are likely
related to the magnetar-like bursts of the pulsar that occurred 7 days before
the Chandra observation, as recently reported from RXTE observations.Comment: Accepted by ApJ, 8 figures, some of them have been scaled down in
resolutio
Ghost of a Shell: Magnetic Fields of Galactic Supershell GSH 006157
We identify a counterpart to a Galactic supershell in diffuse radio
polarisation, and use this to determine the magnetic fields associated with
this object. GSH 006157 has perturbed the polarised emission at
2.3GHz, as observed in the S-band Polarisation All Sky Survey (S-PASS),
acting as a Faraday screen. We model the Faraday rotation over the shell, and
produce a map of Faraday depth over the area across it. Such models require
information about the polarised emission behind the screen, which we obtain
from the Wilkinson Microwave Anisotropy Probe (WMAP), scaled from 23GHz to
2.3GHz, to estimate the synchrotron background behind GSH 006157.
Using the modelled Faraday thickness we determine the magnitude and the
plane-of-the-sky structure of the line-of-sight magnetic field in the shell. We
find a peak line-of-sight field strength of G. Our measurement probes weak magnetic fields
in a low-density regime (number densities of cm) of the ISM,
thus providing crucial information about the magnetic fields in the
partially-ionised phase.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society. 19 pages, 19 figure
The Australia Telescope search for cosmic microwave background anisotropy
In an attempt to detect Cosmic Microwave Background (CMB) anisotropy on arcmin scales, we have made an 8.7-GHz image of a sky region with a resolution of 2 arcmin and high surface brightness sensitivity using the Australia Telescope Compact Array (ATCA) in an ultracompact configuration. The foreground discrete-source confusion was estimated from observations with higher resolution at the same frequency and in a scaled array at a lower frequency. Following the subtraction of the foreground confusion, the field shows no features in excess of the instrument noise. This limits the CMB anisotropy flat-band power to Qflat < 23.6 μ K with 95% confidence; the ATCA filter function (which is available at the website www.atnf.csiro.au/Research/cmbr/cmbr_atca.html) Fl in multipole l-space peaks at leff = 4700 and has half-maximum values at l = 3350 and 6050
A radio-polarisation and rotation measure study of the Gum Nebula and its environment
The Gum Nebula is 36 degree wide shell-like emission nebula at a distance of
only 450 pc. It has been hypothesised to be an old supernova remnant, fossil
HII region, wind-blown bubble, or combination of multiple objects. Here we
investigate the magneto-ionic properties of the nebula using data from recent
surveys: radio-continuum data from the NRAO VLA and S-band Parkes All Sky
Surveys, and H-alpha data from the Southern H-Alpha Sky Survey Atlas. We model
the upper part of the nebula as a spherical shell of ionised gas expanding into
the ambient medium. We perform a maximum-likelihood Markov chain Monte-Carlo
fit to the NVSS rotation measure data, using the H-halpha data to constrain
average electron density in the shell . Assuming a latitudinal background
gradient in RM we find , angular radius
, shell thickness
, ambient magnetic field strength
and warm gas filling factor
. We constrain the local, small-scale (~260 pc)
pitch-angle of the ordered Galactic magnetic field to
, which represents a significant
deviation from the median field orientation on kiloparsec scales
(~-7.2). The moderate compression factor X=6.0\,^{+5.1}_{-2.5} at
the edge of the H-alpha shell implies that the 'old supernova remnant' origin
is unlikely. Our results support a model of the nebula as a HII region around a
wind-blown bubble. Analysis of depolarisation in 2.3 GHz S-PASS data is
consistent with this hypothesis and our best-fitting values agree well with
previous studies of interstellar bubbles.Comment: 33 pages, 16 figures. Accepted by The Astrophysical Journa
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