485 research outputs found
A Ray-Tracing Model of the Vela Pulsar
In the relativistic plasma surrounding a pulsar, a subluminal ordinary-mode
electromagnetic wave will propagate along a magnetic field line. After some
distance, it can break free of the field line and escape the magnetosphere to
reach an observer. We describe a simple model of pulsar radio emission based on
this scenario and find that applying this model to the case of the Vela pulsar
reproduces qualitative characteristics of the observed Vela pulse profile.Comment: 23 pages, 9 figures, accepted for publication in Ap
Discovery of Substructure in the Scatter-Broadened Image of Sgr A*
We have detected substructure within the smooth scattering disk of the
celebrated Galactic Center radio source Sagittarius A* (SgrA*). We observed
this structure at 1.3 cm wavelength with the Very Long Baseline Array together
with the Green Bank Telescope, on baselines of up to 3000 km, long enough to
completely resolve the average scattering disk. Such structure is predicted
theoretically, as a consequence of refraction by large-scale plasma
fluctuations in the interstellar medium. Along with the much-studied
scaling of angular broadening
with observing wavelength , our observations
indicate that the spectrum of interstellar turbulence is shallow, with an inner
scale larger than 300 km. The substructure is consistent with an intrinsic size
of about 1 mas at 1.3 cm wavelength, as inferred from deconvolution of the
average scattering. Further observations of the substructure can set stronger
constraints on the properties of scattering material and on the intrinsic size
of SgrA*. These constraints will guide understanding of effects of
scatter-broadening and emission physics of the black hole, in images with the
Event Horizon Telescope at millimeter wavelengths.Comment: 5 pages, 5 figures, accepted by Astrophysical Journal Letters; minor
corrections to the text and figures are introduce
Effects of Intermittent Emission: Noise Inventory for Scintillating Pulsar B0834+06
We compare signal and noise for observations of the scintillating pulsar
B0834+06, using very-long baseline interferometry and a single-dish
spectrometer. Comparisons between instruments and with models suggest that
amplitude variations of the pulsar strongly affect the amount and distribution
of self-noise. We show that noise follows a quadratic polynomial with flux
density, in spectral observations. Constant coefficients, indicative of
background noise, agree well with expectation; whereas second-order
coefficients, indicative of self-noise, are about 3 times values expected for a
pulsar with constant on-pulse flux density. We show that variations in flux
density during the 10-sec integration account for the discrepancy. In the
secondary spectrum, about 97% of spectral power lies within the pulsar's
typical scintillation bandwidth and timescale; an extended scintillation arc
contains about 3%. For a pulsar with constant on-pulse flux density, noise in
the dynamic spectrum will appear as a uniformly-distributed background in the
secondary spectrum. We find that this uniform noise background contains 95% of
noise in the dynamic spectrum for interferometric observations; but only 35% of
noise in the dynamic spectrum for single-dish observations. Receiver and sky
dominate noise for our interferometric observations, whereas self-noise
dominates for single-dish. We suggest that intermittent emission by the pulsar,
on timescales < 300 microseconds, concentrates self-noise near the origin in
the secondary spectrum, by correlating noise over the dynamic spectrum. We
suggest that intermittency sets fundamental limits on pulsar astrometry or
timing. Accounting of noise may provide means for detection of intermittent
sources, when effects of propagation are unknown or impractical to invert.Comment: 38 pages, 10 figure
Finding agriculture among biodiversity: metadata in practice
The breadth of biodiversity literature available through the Biodiversity Heritage Library (BHL) is potentially of great use to agricultural research. It provides access to literature drawn from across the world, and its archives document the Earth as it was one hundred years ago and more. However, this strength of BHL is also its weakness: the breadth of coverage of BHL can complicate finding relevant literature. In this short paper, we will explore the practical issues arising from attempting to filter out relevant legacy literature to support agricultural research
Interstellar Scintillation of PSR J0437-4715
We studied the turbulence spectrum of the local interstellar plasma in the
direction of PSR J0437-4715, on the basis of our observations and those
reported earlier by others. We combine these data to form a structure function
for the variations of phase along the line of sight to the pulsar. For
observations that did not report them, we infer modulation indices from a
theoretical model. We find that all of the observations fit a power-law
spectrum of turbulence with index n=3.46+/-0.20. We suggest that differences
among reported values for scintillation bandwidth and timescale for this pulsar
arise from differences in observing parameters. We suggest that refractive
effects dominate for this line of sight, with refraction angle about twice the
diffraction angle at 330 MHz observing frequency. We suggest that the
scattering of this pulsar lies in a layer of enhanced turbulence, about 10 pc
from the Sun. We propose that the flux variations of the extragalactic source
PKS 0405-385 arise in the same scattering layer.Comment: 9 pages, 3 figures, 2 table
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