405 research outputs found
The spectrum of small-scale density fluctuations in the solar wind
Interplanetary scintillation observations at frequencies between 74 and 1400 MHz and solar elongations in the range 10â90° are combined to determine the form of the wavenumber spectrum of electron density fluctuations in the range 10â»Âł < k < 10â»Âč/km (where k = 2Ï/λ). The data are best explained by a spectrum in which there is a genuine scale-length; they are not consistent with a simple power-law spectrum. This suggests that turbulence may be less important than some kind of plasma instability in generating small-scale density fluctuations. The relevance of these conclusions to the use of IPS for determining radio source structure is discussed
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
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
The Millisecond Radio Sky: Transients from a Blind Single Pulse Search
We present the results of a search for transient radio bursts of between
0.125 and 32 millisecond duration in two archival pulsar surveys of
intermediate galactic latitudes with the Parkes multibeam receiver. Fourteen
new neutron stars have been discovered, seven of which belong to the recently
identified "rotating radio transients" (RRATs) class. Here we describe our
search methodology, and discuss the new detections in terms of how the RRAT
population relates to the general population of pulsars. The new detections
indicate (1) that the galactic z-distribution of RRATs in the surveys closely
resembles the distribution of pulsars, with objects up to 0.86 kpc from the
galactic plane; (2) where measurable, the RRAT pulse widths are similar to that
of individual pulses from pulsars of similar period, implying a similar beaming
fraction; and (3) our new detections span a variety of nulling fractions, and
thus we postulate that the RRATs may simply be nulling pulsars that are only
"on" for less than a pulse period. Finally, the newly discovered object PSR
J0941-39 may represent a link between pulsars and RRATs. This bizarre object
was discovered as an RRAT, but in follow-up observations often appeared as a
bright (~10 mJy) pulsar with a low nulling fraction. It is obvious therefore
that a neutron star can oscillate between being an RRAT and a pulsar.
Crucially, the sites of the RRAT pulses are coincident with the pulsar's
emission, implying that the two emission mechanisms are linked, and that RRATs
are not just pulsars observed from different orientations.Comment: 13 pages, 9 figures, accepted by MNRA
Selection of radio pulsar candidates using artificial neural networks
Radio pulsar surveys are producing many more pulsar candidates than can be
inspected by human experts in a practical length of time. Here we present a
technique to automatically identify credible pulsar candidates from pulsar
surveys using an artificial neural network. The technique has been applied to
candidates from a recent re-analysis of the Parkes multi-beam pulsar survey
resulting in the discovery of a previously unidentified pulsar.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society. 9 pages, 7 figures, and 1 tabl
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
DiFX: A software correlator for very long baseline interferometry using multi-processor computing environments
We describe the development of an FX style correlator for Very Long Baseline
Interferometry (VLBI), implemented in software and intended to run in
multi-processor computing environments, such as large clusters of commodity
machines (Beowulf clusters) or computers specifically designed for high
performance computing, such as multi-processor shared-memory machines. We
outline the scientific and practical benefits for VLBI correlation, these
chiefly being due to the inherent flexibility of software and the fact that the
highly parallel and scalable nature of the correlation task is well suited to a
multi-processor computing environment. We suggest scientific applications where
such an approach to VLBI correlation is most suited and will give the best
returns. We report detailed results from the Distributed FX (DiFX) software
correlator, running on the Swinburne supercomputer (a Beowulf cluster of
approximately 300 commodity processors), including measures of the performance
of the system. For example, to correlate all Stokes products for a 10 antenna
array, with an aggregate bandwidth of 64 MHz per station and using typical time
and frequency resolution presently requires of order 100 desktop-class compute
nodes. Due to the effect of Moore's Law on commodity computing performance, the
total number and cost of compute nodes required to meet a given correlation
task continues to decrease rapidly with time. We show detailed comparisons
between DiFX and two existing hardware-based correlators: the Australian Long
Baseline Array (LBA) S2 correlator, and the NRAO Very Long Baseline Array
(VLBA) correlator. In both cases, excellent agreement was found between the
correlators. Finally, we describe plans for the future operation of DiFX on the
Swinburne supercomputer, for both astrophysical and geodetic science.Comment: 41 pages, 10 figures, accepted for publication in PAS
Magnetar outbursts: an observational review
Transient outbursts from magnetars have shown to be a key property of their
emission, and one of the main way to discover new sources of this class. From
the discovery of the first transient event around 2003, we now count about a
dozen of outbursts, which increased the number of these strongly magnetic
neutron stars by a third in six years. Magnetar outbursts might involve their
multi-band emission resulting in an increased activity from radio to hard
X-ray, usually with a soft X-ray flux increasing by a factor of 10-1000 with
respect to the quiescent level. A connected X-ray spectral evolution is also
often observed, with a spectral softening during the outburst decay. The flux
decay times vary a lot from source to source, ranging from a few weeks to
several years, as also the decay law which can be exponential-like, a power-law
or even multiple power-laws can be required to model the flux decrease. We
review here on the latest observational results on the multi-band emission of
magnetars, and summarize one by one all the transient events which could be
studied to date from these sources.Comment: 34 pages, 6 figures. Chapter of the Springer Book ASSP 7395
"High-energy emission from pulsars and their systems", proceeding of the Sant
Cugat Forum on Astrophysics (12-16 April 2010). Review updated to January
201
A General Formulation of the Source Confusion Statistics and Application to Infrared Galaxy Surveys
Source confusion has been a long-standing problem in the astronomical
history. In the previous formulation, sources are assumed to be distributed
homogeneously on the sky. This fundamental assumption is not realistic in many
applications. In this work, by making use of the point field theory, we derive
general analytic formulae for the confusion problems with arbitrary
distribution and correlation functions. As a typical example, we apply these
new formulae to the source confusion of infrared galaxies. We first calculate
the confusion statistics for power-law galaxy number counts as a test case.
When the slope of differential number counts, \gamma, is steep, the confusion
limits becomes much brighter and the probability distribution function (PDF) of
the fluctuation field is strongly distorted. Then we estimate the PDF and
confusion limits based on the realistic number count model for infrared
galaxies. The gradual flattening of the slope of the source counts makes the
clustering effect rather mild. Clustering effects result in an increase of the
limiting flux density with \sim 10%. In this case, the peak probability of the
PDF decreases up to \sim 15% and its tail becomes heavier.Comment: ApJ in press, 21 pages, 9 figures, using aastex.cls, emulateapj5.sty.
Abstract abridge
External Electromagnetic Fields of a Slowly Rotating Magnetized Star with Gravitomagnetic Charge
We study Maxwell equations in the external background spacetime of a slowly
rotating magnetized NUT star and find analytical solutions for the exterior
electric fields after separating the equations of electric field into angular
and radial parts in the lowest order approximation. The star is considered
isolated and in vacuum, with dipolar magnetic field aligned with the axis of
rotation. The contribution to the external electric field of star from the NUT
charge is considered in detail.Comment: 6 pages, 2 figures, accepted for publication in Astrophysics and
Space Scienc
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