119 research outputs found
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
The Supernova Remnant CTB104A : Magnetic Field Structure and Interaction with the Environment
We present new, high resolution 1420 and 408 MHz continuum images and HI and
12CO (J=1-0) spectral line maps of the diffuse supernova remnant CTB104A
(G93.7-0.3). Analysis of the complex continuum emission reveals no significant
spectral index variations across the remnant. Three prominences around CTB104A
are found to be related to the SNR, while one extension to the east is
identified as an HII region associated with a background molecular shell. Small
scale polarization and rotation measure (RM) structures are turbulent in
nature, but we find a well-ordered RM gradient across the remnant, extending
from southeast to northwest. This gradient does not agree with the direction of
the global Galactic magnetic field, but does agree with a large-scale RM
anomaly inferred from rotation measure data by Cleg et al. (1992). We show that
the observed morphology of CTB104A is consistent with expansion in a uniform
magnetic field, and this is supported by the observed RM distribution. By
modeling the RM gradient with a simple compression model we have determined the
magnetic field strength within the remnant as Bo ~ 2.3 micro G. We have
identified signatures of the interaction of CTB104A with the surrounding
neutral material, and determined its distance, from the kinematics of the HI
structure encompassing the radio emission, as 1.5 kpc. We also observed clear
breaks in the HI shell that correspond well to the positions of two of the
prominences, indicating regions where hot gas is escaping from the interior of
the SNR.Comment: 7 pages, Latex with aastex and emulateapj5, 12 figures, ApJ accepte
S-band Polarization All Sky Survey (S-PASS): survey description and maps
We present the S-Band Polarization All Sky Survey (S-PASS), a survey of
polarized radio emission over the southern sky at Dec~ taken with
the Parkes radio telescope at 2.3~GHz. The main aim was to observe at a
frequency high enough to avoid strong depolarization at intermediate Galactic
latitudes (still present at 1.4 GHz) to study Galactic magnetism, but low
enough to retain ample Signal-to-Noise ratio (S/N) at high latitudes for
extragalactic and cosmological science. We developed a new scanning strategy
based on long azimuth scans, and a corresponding map-making procedure to make
recovery of the overall mean signal of Stokes and possible, a
long-standing problem with polarization observations. We describe the scanning
strategy, map-making procedure, and validation tests. The overall mean signal
is recovered with a precision better than 0.5\%. The maps have a mean
sensitivity of 0.81 mK on beam--size scales and show clear polarized signals,
typically to within a few degrees of the Galactic plane, with ample S/N
everywhere (the typical signal in low emission regions is 13 mK, and 98.6\% of
the pixels have S/N ). The largest depolarization areas are in the inner
Galaxy, associated with the Sagittarius Arm. We have also computed a Rotation
Measure map combining S-PASS with archival data from the WMAP and Planck
experiments. A Stokes map has been generated, with a sensitivity limited to
the confusion level of 9 mK.Comment: Accepted for publication on MNRAS. Maps are available for download at
the website indicated in the manuscrip
The asymmetric radio remnant of SN 1987A
We present seven years of radio observations of SN 1987A made with the
Australia Telescope Compact Array. At 1.4, 2.4, 4.8 and 8.6 GHz, the flux
density of the radio remnant has increased monotonically since emission was
redetected 1200 days after the explosion. On day 3200, the remnant was
expanding at 2800 +/- 400 km/s, which we interpret as indicating significant
deceleration of the fastest moving ejecta. Since day 1787 the spectral index
has remained constant at alpha = -0.95 +/- 0.04. These observations are all
consistent with the shock having encountered a denser, shocked, component of
the progenitor's stellar wind. At the current rate of expansion, the shock is
expected to encounter the inner optical ring in the year 2006 +/- 3.
Using super-resolution, we have also obtained 9 GHz images of the remnant
(resolution approx 0".5) at four epochs. The emission is distributed around the
rim of a near-circular shell, but has become increasingly asymmetric with time.
There are two "hotspots" to the east and west, aligned along the major axis of
the optical ring. This morphology is most likely indicative of an axisymmetric
circumstellar medium into which the shock is expanding, consistent with present
understanding of the progenitor star and its environment.Comment: 45 pages, LaTeX, including 15 PostScript figures. To appear in "The
Astrophysical Journal", volume 479 (20 Apr 1997
Fourier Modeling of the Radio Torus Surrounding Supernova 1987A
We present detailed Fourier modeling of the radio remnant of Supernova 1987A,
using high-resolution 9 GHz and 18 GHz data taken with the Australia Telescope
Compact Array over the period 1992 to 2008. We develop a parameterized
three-dimensional torus model for the expanding radio shell, in which the
emission is confined to an inclined equatorial belt; our model also
incorporates both a correction for light travel-time effects and an overall
east-west gradient in the radio emissivity. By deriving an analytic expression
for the two-dimensional Fourier transform of the projected three-dimensional
brightness distribution, we can fit our spatial model directly to the
interferometric visibility data. This provides robust estimates to the radio
morphology at each epoch. The best-fit results suggest a constant remnant
expansion at 4000 +/- 400 km/s over the 16-year period covered by the
observations. The model fits also indicate substantial mid-latitude emission,
extending to 40 degree on either side of the equatorial plane. This likely
corresponds to the extra-planar structure seen in H and Ly
emission from the supernova reverse shock, and broadly supports hydrodynamic
models in which the complex circumstellar environment was produced by a
progression of interacting winds from the progenitor. Our model quantifies the
clear asymmetry seen in the radio images: we find that the eastern half of the
radio remnant is consistently ~40 brighter than the western half at all epochs,
which may result from an asymmetry in the ejecta distribution between these two
hemispheres.Comment: accepted by ApJ, 11 figures, some have been scaled down in resolutio
New HI-detected Galaxies in the Zone of Avoidance
We present the first results of a blind HI survey for galaxies in the
southern Zone of Avoidance with a multibeam receiver on the Parkes telescope.
This survey is eventually expected to catalog several thousand galaxies within
Galactic latitude |b|<5 degrees, mostly unrecognised before due to Galactic
extinction and confusion. We present here results of the first three detections
to have been imaged with the Australia Telescope Compact Array (ATCA). The
galaxies all lie near Galactic longitude 325 degrees and were selected because
of their large angular sizes, up to 1.3 degrees. Linear sizes range from 53 to
108 kpc. The first galaxy is a massive 5.7x10^11 solar mass disk galaxy with a
faint optical counterpart, SGC 1511.1--5249. The second is probably an
interacting group of galaxies straddling the Galactic equator. No optical
identification is possible. The third object appears to be an interacting pair
of low column density galaxies, possibly belonging to an extended Circinus or
Centaurus A galaxy group. No optical counterpart has been seen despite the
predicted extinction (A(B) = 2.7 - 4.4 mag) not being excessive. We discuss the
implications of the results, in particular the low HI column densities (~10^19
atoms/sq.cm) found for two of the three galaxies.Comment: 17 pages, 8 figures (Fig.1 in three parts, Fig.5 in two parts). To
appear in Astronomical Journal (Dec 1998). See
http://www.atnf.csiro.au/research/multibea
Radio Emission from the Composite Supernova Remnant G326.3-1.8 (MSH15-56)
High resolution radio observations of the composite supernova remnant (SNR)
G326.3-1.8 or MSH 15-56 with the Australia Telescope Compact Array show details
of both the shell and the bright plerion which is offset about 1/3 of the
distance from the center of the SNR to the shell. The shell appears to be
composed of thin filaments, typical of older shell SNRs. The central part of
the elongated plerion is composed of a bundle of parallel ridges which bulge
out at the ends and form a distinct ring structure on the northwestern end. The
magnetic field with a strength of order 45 microGauss, is directed along the
axis of the ridges but circles around the northwestern ring. This plerion is
large and bright in the radio but is not detected in x-ray or optical
wavelengths. There is, however, a faint hard x-ray feature closer to the shell
outside the plerion. Perhaps if the supernova explosion left a rapidly moving
magnetar with large energy input but initially rapid decay of both relativistic
particles and magnetic field, the observed differences with wavelength could be
explained.Comment: 15 pages, 10 figures, accepted by Ap
Advanced Diagnostics for the Study of Linearly Polarized Emission. II: Application to Diffuse Interstellar Radio Synchrotron Emission
Diagnostics of polarized emission provide us with valuable information on the
Galactic magnetic field and the state of turbulence in the interstellar medium,
which cannot be obtained from synchrotron intensity alone. In Paper I (Herron
et al. 2017b), we derived polarization diagnostics that are rotationally and
translationally invariant in the - plane, similar to the polarization
gradient. In this paper, we apply these diagnostics to simulations of ideal
magnetohydrodynamic turbulence that have a range of sonic and Alfv\'enic Mach
numbers. We generate synthetic images of Stokes and for these
simulations, for the cases where the turbulence is illuminated from behind by
uniform polarized emission, and where the polarized emission originates from
within the turbulent volume. From these simulated images we calculate the
polarization diagnostics derived in Paper I, for different lines of sight
relative to the mean magnetic field, and for a range of frequencies. For all of
our simulations, we find that the polarization gradient is very similar to the
generalized polarization gradient, and that both trace spatial variations in
the magnetoionic medium for the case where emission originates within the
turbulent volume, provided that the medium is not supersonic. We propose a
method for distinguishing the cases of emission coming from behind or within a
turbulent, Faraday rotating medium, and a method to partly map the rotation
measure of the observed region. We also speculate on statistics of these
diagnostics that may allow us to constrain the physical properties of an
observed turbulent region.Comment: 34 pages, 25 figures, accepted for publication in Ap
A high-resolution radio survey of the Vela supernova remnant
This paper presents a high-resolution radio continuum (843 MHz) survey of the
Vela supernova remnant. The contrast between the structures in the central
pulsar-powered nebula of the remnant and the synchrotron radiation shell allows
the remnant to be identified morphologically as a member of the composite
class. The data are the first of a composite remnant at spatial scales
comparable with those available for the Cygnus Loop and the Crab Nebula, and
make possible a comparison of radio, optical and soft X-ray emission from the
resolved shell filaments. The survey, made with the Molonglo Observatory
Synthesis Telescope, covers an area of 50 square degrees at a resolution of
43'' x 60'', while imaging structures on scales up to 30'.Comment: 18 pages, 7 jpg figures (version with ps figures at
http://astro.berkeley.edu/~dbock/papers/); AJ, in pres
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