78 research outputs found
A fast and exact -stacking and -projection hybrid algorithm for wide-field interferometric imaging
The standard wide-field imaging technique, the -projection, allows
correction for wide-fields of view for non-coplanar radio interferometric
arrays. However, calculating exact corrections for each measurement has not
been possible due to the amount of computation required at high resolution and
with the large number of visibilities from current interferometers. The
required accuracy and computational cost of these corrections is one of the
largest unsolved challenges facing next generation radio interferometers such
as the Square Kilometre Array. We show that the same calculation can be
performed with a radially symmetric -projection kernel, where we use one
dimensional adaptive quadrature to calculate the resulting Hankel transform,
decreasing the computation required for kernel generation by several orders of
magnitude, whilst preserving the accuracy. We confirm that the radial
-projection kernel is accurate to approximately 1% by imaging the
zero-spacing with an added -term. We demonstrate the potential of our
radially symmetric -projection kernel via sparse image reconstruction, using
the software package PURIFY. We develop a distributed -stacking and
-projection hybrid algorithm. We apply this algorithm to individually
correct for non-coplanar effects in 17.5 million visibilities over a by
degree field of view MWA observation for image reconstruction. Such a
level of accuracy and scalability is not possible with standard -projection
kernel generation methods. This demonstrates that we can scale to a large
number of measurements with large image sizes whilst still maintaining both
speed and accuracy.Comment: 9 Figures, 19 Pages. Accepted to Ap
Using head-tail galaxies to constrain the intracluster magnetic field: an in-depth study of PKS J0334-3900
We present a multi-wavelength study of the radio galaxy PKS J0334-3900 at the
centre of Abell 3135. The spectro-polarimetric radio observations are combined
with spectroscopic optical and X-ray data to illustrate the use of Head-Tail
radio galaxies to reveal properties of the intracluster medium. ATCA
observations at 1.4, 2.5, 4.6 & 8.6 GHz are presented with a detailed analysis
of the morphology and spectral indices giving physical parameters to constrain
the dynamical history of the galaxy. Using these constraints we produce a
simulation of PKS J0334-3900. We find that this Head-Tail morphology can be
induced via a combination of orbital motion due to a binary companion and
relative motion through the ICM. New Chandra images of A3135 are presented from
which we obtain a cluster electron density of n_(e,0) = (1.06 +/- 0.11 x
10^(-3) cm^(-3), a global temperature of 2.4 ^(+0.51)_(-0.38) keV and a lower
limit to the radio jet power of PKS J0334-3900 of 1.6 x 10^(44) erg/s. A new
redshift analysis of the cluster from available spectroscopic data shows A3135
to be comprised of galaxies with 0.058 < z < 0.066 and gives a new mean cluster
redshift of 0.06228 +/- 0.00015. We also uncovered a background subgroup
between 0.066 < z < 0.070. Stokes Q and U data of Abell 3135 were used to
obtain rotation measure values along the line of sight to PKS J0334-3900. Using
our simulation we are able to infer the distance between the jets to be 154 +/-
16 kpc, which when combined with the difference in rotation measure between the
jets provides a novel new way to estimate the average magnetic field within a
cluster. A lower limit to the cluster B-field was calculated to be 0.09 +/-
0.03 micro Gauss. We show observations of Head-Tail galaxies can be used to
infer information on the cluster environment, showing them to be an important
class of objects in next generation all sky surveys.Comment: 17 pages, 15 figures, accepted to MNRAS. Version 2 corrects a mistake
in the average RM between the jets (and hence the lower estimate of the
cluster B-field) which was found during proofin
Peer review role-playing as a method of teaching senior undergraduate science
We present a case study of the use of role-playing as a method of enhancing course delivery in senior undergraduate science courses. Specifically we examine the use of simulating peer review of scientific proposals for time on major international telescopes in a third year astrophysics course at the University of Tasmania. We find that 90% of participating students found the experience to be both enjoyable and a good method for learning and that 100% of students felt that the exercise had aided their understanding of the course material. Furthermore 80% of students felt that the role-playing had enhanced their understanding of scientific decision making while 70% believed that they had learnt to apply principles from the course in new situations
Radio Sources in the NCP Region Observed with the 21 Centimeter Array
We present a catalog of 624 radio sources detected around the North Celestial
Pole (NCP) with the 21 Centimeter Array (21CMA), a radio interferometer
dedicated to the statistical measurement of the epoch of reionization (EoR).
The data are taken from a 12 h observation made on 2013 April 13, with a
frequency coverage from 75 to 175 MHz and an angular resolution of ~ 4 arcmin.
The catalog includes flux densities at eight sub-bands across the 21CMA
bandwidth and provides the in-band spectral indices for the detected sources.
To reduce the complexity of interferometric imaging from the so-called "w" term
and ionospheric effects, the present analysis are restricted to the east-west
baselines within 1500 m only. The 624 radio sources are found within 5 degrees
around the NCP down to ~ 0.1 Jy. Our source counts are compared, and also
exhibit a good agreement, with deep low-frequency observations made recently
with the GMRT and MWA. In particular, for fainter radio sources below ~ 1 Jy,
we find a flattening trend of source counts towards lower frequencies. While
the thermal noise (~0.4 mJy) is well controlled to below the confusion limit,
the dynamical range (~10^4) and sensitivity of current 21CMA imaging is largely
limited by calibration and deconvolution errors, especially the grating lobes
of very bright sources, such as 3C061.1, in the NCP field which result from the
regular spacings of the 21CMA. We note that particular attention should be paid
to the extended sources, and their modeling and removals may constitute a large
technical challenge for current EoR experiments. Our analysis may serve as a
useful guide to design of next generation low-frequency interferometers like
the Square Kilometre Array.Comment: 16 pages, 21 figures, 7 tables, 1 machine readable table, accepted
for publication in Ap
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