357 research outputs found
Radio Scintillation due to Discontinuities in the Interstellar Plasma Density
We develop the theory of interstellar scintillation as caused by an irregular
plasma having a power-law spatial density spectrum with a spectral exponent of
4 corresponding to a medium with abrupt changes in its density. An ``outer
scale'' is included in the model representing the typical scale over which the
density of the medium remains uniform. Such a spectrum could be used to model
plasma shock fronts in supernova remnants or other plasma discontinuities. We
investigate and develop equations for the decorrelation bandwidth of
diffractive scintillations and the refractive scintillation index and compare
our results with pulsar measurements. We consider both a medium concentrated in
a thin layer and an extended irregular medium. We conclude that the
discontinuity model gives satisfactory agreement for many diffractive
measurements, in particular the VLBI meaurements of the structure function
exponent between 5/3 and 2. However, it gives less satisfactory agreement for
the refractive scintillation index than does the Kolmogorov turbulence
spectrum. The comparison suggests that the medium consists of a pervasive
background distribution of turbulence embedded with randomly placed discrete
plasma structures such as shocks or HII regions. This can be modeled by a
composite spectrum following the Kolmogorov form at high wavenumbers and
steepening at lower wavenumbers corresponding to the typical (inverse) size of
the discrete structures. Such a model can also explain the extreme scattering
events. However, lines of sight through the enhanced scattering prevalent at
low galactic latitudes are accurately described by the Kolmogorov spectrum in
an extended medium and do not appear to have a similar low-wavenumber
steepening.Comment: Accpeted for ApJ vol 531, March 200
Electric field representation of pulsar intensity spectra
Pulsar dynamic spectra exhibit high visibility fringes arising from
interference between scattered radio waves. These fringes may be random or
highly ordered patterns, depending on the nature of the scattering or
refraction. Here we consider the possibility of decomposing pulsar dynamic
spectra -- which are intensity measurements -- into their constituent scattered
waves, i.e. electric field components. We describe an iterative method of
achieving this decomposition and show how the algorithm performs on data from
the pulsar B0834+06. The match between model and observations is good, although
not formally acceptable as a representation of the data. Scattered wave
components derived in this way are immediately useful for qualitative insights
into the scattering geometry. With some further development this approach can
be put to a variety of uses, including: imaging the scattering and refracting
structures in the interstellar medium; interstellar interferometric imaging of
pulsars at very high angular resolution; and mitigating pulse arrival time
fluctuations due to interstellar scattering.Comment: 7 Pages, 2 Figures, revised version, accepted by MNRA
Interstellar Scintillation Observations of 146 Extragalactic Radio Sources
From 1979--1996 the Green Bank Interferometer was used by the Naval Research
Laboratory to monitor the flux density from 146 compact radio sources at
frequencies near 2 and 8 GHz. We filter the ``light curves'' to separate
intrinsic variations on times of a year or more from more rapid interstellar
scintilation (ISS) on times of 5--50 d. Whereas the intrinsic variation at 2
GHz is similar to that at 8 GHz (though diminished in amplitude), the ISS
variation is much stronger at 2 than at 8 GHz. We characterize the ISS
variation by an rms amplitude and a timescale and examine the statistics of
these parameters for the 121 sources with significant ISS at 2 GHz. We model
the scintillations using the NE2001 Galactic electron model assuming the
sources are brightness-limited.
We find the observed rms amplitude to be in general agreement with the model,
provided that the compact components of the sources have about 50% of their
flux density in a component with maximum brightness temperatures
--K. Thus our results are consistent with cm-wavelength VLBI
studies of compact AGNs, in that the maximum brightness temperatures found are
consistent with the inverse synchrotron limit at K, boosted
in jet configurations by Doppler factors up to about 20. The average of the
observed 2 GHz ISS timescales is in reasonable agreement with the model at
Galactic latitudes above about 10\de. At lower latitudes the observed
timescales are too fast, suggesting that the transverse plasma velocity
increases more than expected beyond about 1 kpc.Comment: 32 pages, 16 figures. Submitted to Ap
Modeling of Interstellar Scintillation Arcs from Pulsar B1133+16
The parabolic arc phenomenon visible in the Fourier analysis of the
scintillation spectra of pulsars provides a new method of investigating the
small scale structure in the ionized interstellar medium (ISM). We report
archival observations of the pulsar B1133+16 showing both forward and reverse
parabolic arcs sampled over 14 months. These features can be understood as the
mutual interference between an assembly of discrete features in the scattered
brightness distribution. By model-fitting to the observed arcs at one epoch we
obtain a ``snap-shot'' estimate of the scattered brightness, which we show to
be highly anisotropic (axial ratio >10:1), to be centered significantly off
axis and to have a small number of discrete maxima, which are coarser the
speckle expected from a Kolmogorov spectrum of interstellar plasma density. The
results suggest the effects of highly localized discrete scattering regions
which subtend 0.1-1 mas, but can scatter (or refract) the radiation by angles
that are five or more times larger.Comment: 14 pages, 4 figures, submitted to Astrophysical Journa
Long-term monitoring of Molonglo calibrators
Before and after every 12 hour synthesis observation, the Molonglo
Observatory Synthesis Telescope (MOST) measures the flux densities of ~5
compact extragalactic radio sources, chosen from a list of 55 calibrators. From
1984 to 1996, the MOST made some 58 000 such measurements. We have developed an
algorithm to process this dataset to produce a light curve for each source
spanning this thirteen year period. We find that 18 of the 55 calibrators are
variable, on time scales between one and ten years. There is the tendency for
sources closer to the Galactic Plane to be more likely to vary, which suggests
that the variability is a result of refractive scintillation in the Galactic
interstellar medium. The sources with the flattest radio spectra show the
highest levels of variability, an effect possibly resulting from differing
orientations of the radio axes to the line of sight.Comment: 18 pages, 9 embedded EPS files. To appear in Publications of the
Astronomical Society of Australia. Data available electronically at
http://www.physics.usyd.edu.au/astrop/scan
The stationary phase point method for transitional scattering: diffractive radio scintillation for pulsar
The stationary phase point (SPP) method in one-dimensional case is introduced
to treat the diffractive scintillation. From weak scattering, where the SPP
number N=1, to strong scattering (N1), via transitional scattering regime
(N2,3), we find that the modulation index of intensity experiences the
monotonically increasing from 0 to 1 with the scattering strength,
characterized by the ratio of Fresnel scale \rf to diffractive scale
\rdiff.Comment: Hanas Meeting paper, appear in ChJAA, 2006, 6, Su
Theory of Parabolic Arcs in Interstellar Scintillation Spectra
Our theory relates the secondary spectrum, the 2D power spectrum of the radio
dynamic spectrum, to the scattered pulsar image in a thin scattering screen
geometry. Recently discovered parabolic arcs in secondary spectra are generic
features for media that scatter radiation at angles much larger than the rms
scattering angle. Each point in the secondary spectrum maps particular values
of differential arrival-time delay and fringe rate (or differential Doppler
frequency) between pairs of components in the scattered image. Arcs correspond
to a parabolic relation between these quantities through their common
dependence on the angle of arrival of scattered components. Arcs appear even
without consideration of the dispersive nature of the plasma. Arcs are more
prominent in media with negligible inner scale and with shallow wavenumber
spectra, such as the Kolmogorov spectrum, and when the scattered image is
elongated along the velocity direction. The arc phenomenon can be used,
therefore, to constrain the inner scale and the anisotropy of scattering
irregularities for directions to nearby pulsars. Arcs are truncated by finite
source size and thus provide sub micro arc sec resolution for probing emission
regions in pulsars and compact active galactic nuclei. Multiple arcs sometimes
seen signify two or more discrete scattering screens along the propagation
path, and small arclets oriented oppositely to the main arc persisting for long
durations indicate the occurrence of long-term multiple images from the
scattering screen.Comment: 22 pages, 11 figures, submitted to the Astrophysical Journa
Dual-Frequency Observations of 140 Compact, Flat-Spectrum Active Galactic Nuclei for Scintillation-Induced Variability
The 4.9 GHz Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey
detected a drop in Interstellar Scintillation (ISS) for sources at redshifts z
> 2, indicating an apparent increase in angular diameter or a decrease in flux
density of the most compact components of these sources, relative to their
extended emission. This can result from intrinsic source size effects or
scatter broadening in the Intergalactic Medium (IGM), in excess of the expected
(1+z)^0.5 angular diameter scaling of brightness temperature limited sources
due to cosmological expansion. We report here 4.9 GHz and 8.4 GHz observations
and data analysis for a sample of 140 compact, flat-spectrum sources which may
allow us to determine the origin of this angular diameter-redshift relation by
exploiting their different wavelength dependences. In addition to using ISS as
a cosmological probe, the observations provide additional insight into source
morphologies and the characteristics of ISS. As in the MASIV Survey, the
variability of the sources is found to be significantly correlated with
line-of-sight H-alpha intensities, confirming its link with ISS. For 25
sources, time delays of about 0.15 to 3 days are observed between the
scintillation patterns at both frequencies, interpreted as being caused by a
shift in core positions when probed at different optical depths. Significant
correlation is found between ISS amplitudes and source spectral index; in
particular, a large drop in ISS amplitudes is observed at spectral indices of <
-0.4 confirming that steep spectrum sources scintillate less. We detect a
weakened redshift dependence of ISS at 8.4 GHz over that at 4.9 GHz, with the
mean variance at 4-day timescales reduced by a factor of 1.8 in the z > 2
sources relative to the z < 2 sources, as opposed to the factor of 3 decrease
observed at 4.9 GHz. This suggests scatter broadening in the IGM.Comment: 30 pages, 14 figures, accepted for publication in the Astronomical
Journa
Interferometric Phase Calibration Sources in the Declination Range 0deg to -30deg
We present a catalog of 321 compact radio sources in the declination range
0deg > delta > -30deg. The positions of these sources have been measured with a
two-dimensional rms accuracy of 35 milliarcseconds using the NRAO Very Large
Array. Each source has a peak flux density >50 mJy at 8.4 GHz. We intend for
this catalog to be used mainly for selection of phase calibration sources for
radio interferometers, although compact radio sources have other scientific
uses.Comment: 9 pages. To appear in ApJS. Catalog (Table 3) is abbreviated in
printed version. Complete catalog available at
ftp://ftp.aoc.nrao.edu/pub/staff/jwrobel/WPW2003_ApJS.tx
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