236 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
Robust correlators
Radio frequency interference (RFI) already limits the sensitivity of existing
radio telescopes in several frequency bands and may prove to be an even greater
obstacle for future generation instruments to overcome. I aim to create a
structure of radio astronomy correlators which will be statistically stable
(robust) in the presence of interference. A statistical analysis of the mixture
of system noise + signal noise + RFI is proposed here which could be
incorporated into the block diagram of a correlator. Order and rank statistics
are especially useful when calculated in both temporal and frequency domains.
Several new algorithms of robust correlators are proposed and investigated
here. Computer simulations and processing of real data demonstrate the efficacy
of the proposed algorithms
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
Statistically Stable Estimates of Variance in Radioastronomical Observations as Tools for RFI Mitigation
A selection of statistically stable (robust) algorithms for data variance
calculating has been made. Their properties have been analyzed via computer
simulation. These algorithms would be useful if adopted in radio astronomy
observations in the presence of strong sporadic radio frequency interference
(RFI). Several observational results have been presented here to demonstrate
the effectiveness of these algorithms in RFI mitigation
The kinematics of the bi-lobal supernova remnant G 65.3+5.7 - Paper II
Further deep, narrow-band images in the light of [O III] 5007 A have been
added to the previous mosaic of the faint galactic supernova remnant G
65.3+5.7. Additionally longslit spatially resolved [O III] 5007 A line profiles
have been obtained at sample positions using the Manchester Echelle
Spectrometer at the San Pedro Martir observatory. The remnant is shown to be
predominantly bi-lobal with an EW axis for this structure. However, a faint
additional northern lobe has now been revealed.
Splitting of the profiles along the slit lengths, when extrapolated to the
remnant's centre, although uncertain suggests that the expansion velocity of
this remnant is between 124 and 187 km/s ie much lower than the 400 km/s
previously predicted for the forward shock velocity from the X-ray emission.
An expansion proper motion measurement of 2.1+-0.4 arcsec in 48 years for the
remnant's filamentary edge in the light of Halpha+[N II] has also been made.
When combined with an expansion velocity of ~155 km/s, a distance of ~800 pc to
G 65.3+5.7 is derived.
Several possibilities are considered for the large difference in the
expansion velocity measured here and the 400 km/s shock velocity required to
generate the X-ray emission. It is also suggested that the morphology of the
remnant may be created by a tilt in the galactic magnetic field in this
vicinity.Comment: 10 pages, 5 figures, accepted for publication in A&
The Evolution of Adiabatic Supernova Remnants in a Turbulent, Magnetized Medium
(Abridged) We present the results of three dimensional calculations for the
MHD evolution of an adiabatic supernova remnant in both a uniform and turbulent
interstellar medium using the RIEMANN framework of Balsara. In the uniform
case, which contains an initially uniform magnetic field, the density structure
of the shell remains largely spherical, while the magnetic pressure and
synchrotron emissivity are enhanced along the plane perpendicular to the field
direction. This produces a bilateral or barrel-type morphology in synchrotron
emission for certain viewing angles. We then consider a case with a turbulent
external medium as in Balsara & Pouquet, characterized by .
Several important changes are found. First, despite the presence of a uniform
field, the overall synchrotron emissivity becomes approximately spherically
symmetric, on the whole, but is extremely patchy and time-variable, with
flickering on the order of a few computational time steps. We suggest that the
time and spatial variability of emission in early phase SNR evolution provides
information on the turbulent medium surrounding the remnant. The
shock-turbulence interaction is also shown to be a strong source of
helicity-generation and, therefore, has important consequences for magnetic
field generation. We compare our calculations to the Sedov-phase evolution, and
discuss how the emission characteristics of SNR may provide a diagnostic on the
nature of turbulence in the pre-supernova environment.Comment: ApJ, in press, 5 color figure
Spatially resolved X-ray spectroscopy and modeling of the nonthermal emission of the PWN in G0.9+0.1
We performed a spatially resolved spectral X-ray study of the pulsar wind
nebula (PWN) in the supernova remnant G0.9+0.1. Furthermore we modeled its
nonthermal emission in the X-ray and very high energy (VHE, E > 100 GeV)
gamma-ray regime. Using Chandra ACIS-S3 data, we investigated the east-west
dependence of the spectral properties of G0.9+0.1 by calculating hardness
ratios. We analyzed the EPIC-MOS and EPIC-pn data of two on-axis observations
of the XMM-Newton telescope and extracted spectra of four annulus-shaped
regions, centered on the region of brightest emission of the source. A radially
symmetric leptonic model was applied in order to reproduce the observed X-ray
emission of the inner part of the PWN. Using the optimized model parameter
values obtained from the X-ray analysis, we then compared the modeled inverse
Compton (IC) radiation with the published H.E.S.S. gamma-ray data. The spectral
index within the four annuli increases with growing distance to the pulsar,
whereas the surface brightness drops. With the adopted model we are able to
reproduce the characteristics of the X-ray spectra. The model results for the
VHE gamma radiation, however, strongly deviate from the H.E.S.S. data.Comment: 8 pages, 7 figures, accepted for publication in Astronomy &
Astrophysic
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
A Catalog of HI Clouds in the Large Magellanic Cloud
A 21 cm neutral hydrogen interferometric survey of the Large Magellanic Cloud
(LMC) combined with the Parkes multi-beam HI single-dish survey clearly shows
that the HI gas is distributed in the form of clumps or clouds. The HI clouds
and clumps have been identified using a thresholding method with three separate
brightness temperature thresholds (). Each catalog of HI cloud candidates
shows a power law relationship between the sizes and the velocity dispersions
of the clouds roughly following the Larson Law scaling , with steeper indices associated with dynamically hot regions. The
clouds in each catalog have roughly constant virial parameters as a function
mass suggesting that that the clouds are all in roughly the same dynamical
state, but the values of the virial parameter are significantly larger than
unity showing that turbulent motions dominate gravity in these clouds. The mass
distribution of the clouds is a power law with differential indices between
-1.6 and -2.0 for the three catalogs. In contrast, the distribution of mean
surface densities is a log-normal distribution.Comment: 24 pages, 15 figures, ApJS, in pres
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