1,044 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
Ultra-High Resolution Intensity Statistics of a Scintillating Source
We derive the distribution of flux density of a compact source exhibiting
strong diffractive scintillation. Our treatment accounts for arbitrary spectral
averaging, spatially-extended source emission, and the possibility of intrinsic
variability within the averaging time, as is typical for pulsars. We also
derive the modulation index and present a technique for estimating the
self-noise of the distribution, which can be used to identify amplitude
variations on timescales shorter than the spectral accumulation time. Our
results enable a for direct comparison with ultra-high resolution observations
of pulsars, particularly single-pulse studies with Nyquist-limited resolution,
and can be used to identify the spatial emission structure of individual pulses
at a small fraction of the diffractive scale.Comment: 14 Pages, 4 Figures, accepted for publication in Ap
Size of the Vela Pulsar's Emission Region at 18 cm Wavelength
We present measurements of the linear diameter of the emission region of the
Vela pulsar at observing wavelength lambda=18 cm. We infer the diameter as a
function of pulse phase from the distribution of visibility on the
Mopra-Tidbinbilla baseline. As we demonstrate, in the presence of strong
scintillation, finite size of the emission region produces a characteristic
W-shaped signature in the projection of the visibility distribution onto the
real axis. This modification involves heightened probability density near the
mean amplitude, decreased probability to either side, and a return to the
zero-size distribution beyond. We observe this signature with high statistical
significance, as compared with the best-fitting zero-size model, in many
regions of pulse phase. We find that the equivalent full width at half maximum
of the pulsar's emission region decreases from more than 400 km early in the
pulse to near zero at the peak of the pulse, and then increases again to
approximately 800 km near the trailing edge. We discuss possible systematic
effects, and compare our work with previous results
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
Small-Scale Variations of HI Spectra from Interstellar Scintillatio
I suggest that radio-wave scattering by the interstellar plasma, in
combination with subsonic gradients in the Doppler velocity of interstellar HI,
is responsible for the observed small-scale variation in HI absorption spectra
of pulsars. Velocity gradients on the order of 0.05 to 0.3 km/s across 1 AU can
produce the observed variations. I suggest observational tests to distinguish
between this model and the traditional picture of small-scale opacity
variations from cloudlets.Comment: 24 pages, 2 figures, Latex, uses AASTe
On the Ionisation of Warm Opaque Interstellar Clouds and the Intercloud Medium
In this paper we use a number of observations to construct an integrated
picture of the ionisation in the interiors of quiescent warm opaque
interstellar clouds and in the intercloud medium (ICM) outside dense HII
regions and hot dilute bubbles. Our main conclusion is that within 1kpc
of the sun the ionisation rate of hydrogen per unit volume in both the
interiors of such clouds and in the ICM is independent of the local density of
neutral hydrogen, and varies with position by less than 20 per cent.
These conclusions strongly favour the decaying neutrino hypothesis for the
ionisation of the interstellar medium in these regions.
Our analysis is based on a variety of observations, of which the most
remarkable is the discovery by Spitzer and Fitzpatrick (1993) that, in the four
slowly moving clouds along the line of sight to the halo star HD93521, the
column densities of both SII and CII, which individually range over a
factor 4, are proportional to the column density of HI to within 20
per cent. This proportionality is used to show that the free electrons exciting
the CII to CII are located mainly in the interiors of the clouds, rather
than in their skins, despite the large opacity of the clouds to Lyman continuum
radiation. The same conclusion also follows more unambiguously from the low
value of the H flux in this direction which was found by Reynolds
(1996) in unpublished observations.
These results are then used, in conjunction with observations of three pulsar
parallaxes and dispersion measures, and with data on HeI, NII and OI line
emissions, to constrain the ionisation of H, He, N and O and the flux of Lyman
continuum photons from O stars in the ICM.Comment: 16 pages, no figures, Latex fil
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