230 research outputs found
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
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
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
Neutron star composition in strong magnetic fields
We study the problem of neutron star composition in the presence of a strong
magnetic field. The effects of the anomalous magnetic moments of both nucleons
and electrons are investigated in relativistic mean field calculations for a
-equilibrium system. Since neutrons are fully spin polarized in a large
field, generally speaking, the proton fraction can never exceed the field free
case. An extremely strong magnetic field may lead to a pure neutron matter
instead of a proton-rich matter.Comment: 12 pages, 3 postscript files include
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
Resolving the Radio Source Background: Deeper Understanding Through Confusion
We used the Karl G. Jansky Very Large Array (VLA) to image one primary beam
area at 3 GHz with 8 arcsec FWHM resolution and 1.0 microJy/beam rms noise near
the pointing center. The P(D) distribution from the central 10 arcmin of this
confusion-limited image constrains the count of discrete sources in the 1 <
S(microJy/beam) < 10 range. At this level the brightness-weighted differential
count S^2 n(S) is converging rapidly, as predicted by evolutionary models in
which the faintest radio sources are star-forming galaxies; and ~96$% of the
background originating in galaxies has been resolved into discrete sources.
About 63% of the radio background is produced by AGNs, and the remaining 37%
comes from star-forming galaxies that obey the far-infrared (FIR) / radio
correlation and account for most of the FIR background at lambda = 160 microns.
Our new data confirm that radio sources powered by AGNs and star formation
evolve at about the same rate, a result consistent with AGN feedback and the
rough correlation of black hole and bulge stellar masses. The confusion at
centimeter wavelengths is low enough that neither the planned SKA nor its
pathfinder ASKAP EMU survey should be confusion limited, and the ultimate
source detection limit imposed by "natural" confusion is < 0.01 microJy at 1.4
GHz. If discrete sources dominate the bright extragalactic background reported
by ARCADE2 at 3.3 GHz, they cannot be located in or near galaxies and most are
< 0.03 microJy at 1.4 GHz.Comment: 28 pages including 16 figures. ApJ accepted for publicatio
Multi-frequency VLBA Observations of the Compact Double B2 2050+36: Constraints on Interstellar Scattering Revisited
We present multi-frequency observations with the Very Long Baseline Array of
the compact double radio source B2 2050+36. Our observations are at 0.33, 0.61,
1.67, 2.3, and 8.4 GHz, with the 0.61 GHz observations forming the third epoch
of observation of this source at that frequency. At 0.61 GHz, the structure of
B2 2050+36 is dominated by two components 56 mas apart. Within the
uncertainties of the various measurements, this separation has remained
unchanged for the past 16 years. Any differential image wander caused by
refractive interstellar scattering is less than 4 mas. Both the lack of
differential image wander and the frequency dependence of the angular diameter
of B2 2050+36 below 1 GHz indicate that the electron density power spectrum
along this line of sight has a spectral index near the Kolmogorov value, with a
value of 4 being highly unlikely. We conclude that diffractive scattering
dominates along this line of sight; results in the literature indicate that
this conclusion also holds true for the line of sight to the pulsar PSR
B2020+28 (8.7 deg. from B2 2050+36). Comparison of our 1.67 GHz observations
with those obtained 21 years previously place a limit on the projected linear
separation velocity of the two components of c.Comment: 19 pages LaTeX2e with AASTeX 5, 3 figures in 7 PostScript files;
accepted for publication in the Ap
The High Time Resolution Universe Pulsar Survey I: System configuration and initial discoveries
We have embarked on a survey for pulsars and fast transients using the
13-beam Multibeam receiver on the Parkes radio telescope. Installation of a
digital backend allows us to record 400 MHz of bandwidth for each beam, split
into 1024 channels and sampled every 64 us. Limits of the receiver package
restrict us to a 340 MHz observing band centred at 1352 MHz. The factor of
eight improvement in frequency resolution over previous multibeam surveys
allows us to probe deeper into the Galactic plane for short duration signals
such as the pulses from millisecond pulsars. We plan to survey the entire
southern sky in 42641 pointings, split into low, mid and high Galactic latitude
regions, with integration times of 4200, 540 and 270 s respectively.
Simulations suggest that we will discover 400 pulsars, of which 75 will be
millisecond pulsars. With ~30% of the mid-latitude survey complete, we have
re-detected 223 previously known pulsars and discovered 27 pulsars, 5 of which
are millisecond pulsars. The newly discovered millisecond pulsars tend to have
larger dispersion measures than those discovered in previous surveys, as
expected from the improved time and frequency resolution of our instrument.Comment: Updated author list. 10 pages, 7 figures. For publication in MNRA
Neutron Stars in Teleparallel Gravity
In this paper we deal with neutron stars, which are described by a perfect
fluid model, in the context of the teleparallel equivalent of general
relativity. We use numerical simulations to find the relationship between the
angular momentum of the field and the angular momentum of the source. Such a
relation was established for each stable star reached by the numerical
simulation once the code is fed with an equation of state, the central energy
density and the ratio between polar and equatorial radii. We also find a regime
where linear relation between gravitational angular momentum and moment of
inertia (as well as angular velocity of the fluid) is valid. We give the
spatial distribution of the gravitational energy and show that it has a linear
dependence with the squared angular velocity of the source.Comment: 19 pages, 14 figures. arXiv admin note: text overlap with
arXiv:1206.331
Isolated neutron stars and studies of their interiors
In these lectures presented at Baikal summer school on physics of elementary
particles and astrophysics 2011, I present a wide view of neutron star
astrophysics with special attention paid to young isolated compact objects and
studies of the properties of neutron star interiors using astronomical methods.Comment: 28 pages, lecture notes for the Baikal-2011 summer school on physics
of elementary particles and astrophysic
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