823 research outputs found
Low Frequency Interstellar Scattering and Pulsar Observations
Radio astronomy at frequencies from 2 to 30 MHz challenges time tested methods for extracting usable information from observations. One fundamental reason for this is that propagation effects due to the magnetoionic ionosphere, interplanetary medium, and interstellar matter (ISM) increase strongly with wavelength. The problems associated with interstellar scattering off of small scale irregularities in the electron density are addressed. What is known about interstellar scattering is summarized on the basis of high frequency observations, including scintillation and temporal broadening of pulsars and angular broadening of various galactic and extragalactic radio sources. Then those high frequency phenomena are addressed that are important or detectable at low frequencies. The radio sky becomes much simpler at low frequencies, most pulsars will not be seen as time varying sources, intensity variations will be quenched or will occur on time scales much longer than a human lifetime, and many sources will be angularly broadened and/or absorbed into the noise. Angular broadening measurements will help delineate the galactic distribution and power spectrum of small scale electron density irregularities
Constraints on cosmic ray propagation in the galaxy
The goal was to derive a more detailed picture of magnetohydrodynamic turbulence in the interstellar medium and its effects on cosmic ray propagation. To do so, radio astronomical observations (scattering and Faraday rotation) were combined with knowledge of solar system spacecraft observations of MHD turbulence, simulations of wave propagation, and modeling of the galactic distribution to improve the knowledge. A more sophisticated model was developed for the galactic distribution of electron density turbulence. Faraday rotation measure data was analyzed to constrain magnetic field fluctuations in the ISM. VLBI observations were acquired of compact sources behind the supernova remnant CTA1. Simple calculations were made about the energies of the turbulence assuming a direct link between electron density and magnetic field variations. A simulation is outlined of cosmic ray propagation through the galaxy using the above results
Spin Evolution of Pulsars with Weakly Coupled Superfluid Interiors
We discuss the spin evolution of pulsars in the case where a superfluid
component of the star is coupled to the observable crust on long, spindown
timescales. The momentum transfer from the superfluid interior results in an
apparent decay of the external torque and, after a dramatic increase, to an
asymptotic decrease of the generic value of the braking index, e.g. , to
values if the magnetic field of the star does not decay over its
lifetime. In the case where an exponential decay of the magnetic field towards
a residual value occurs, the star undergoes a spin-up phase after which it
could emerge in the millisecond sector of the - diagram.Comment: 11 pages, 3 figures, Latex, uses aaspp4.sty; ApJ in pres
Flying The Open Skies: An Analysis and Historical Perspective of the U.S.-Netherlands Bilateral Air Transport Agreement of September 4, 1992.
Vortex-interface interactions and generation of glitches in pulsars
We show that the crust-core interface in neutron stars acts as a potential
barrier to the peripheral neutron vortices approaching the interface in the
model in which these are coupled to the proton vortex clusters. This elementary
barrier because of the interaction of vortex magnetic flux with the Meissner
currents set up by the crustal magnetic field at the interface. The dominant
part of the force is derived from to the cluster-interface interaction. As a
result of the stopping of the continuous neutron vortex current through the
interface, angular momentum is stored in the superfluid layers in the vicinity
of the crust-core interface during the interglitch period. Discontinuous
annihilation of proton vortices on the boundary restores the neutron vortex
current and spins up the observable crust on short time-scales, leading to a
glitch in the spin characteristics of a pulsar.Comment: 12 pages, 3 figures. Final version to appear in MNRAS; includes a new
discussion of the problem in the spherical geometry; some rewording;
references revise
Interstellar Scattering Towards the Galactic Center as Probed by OH/IR Stars
Angular broadening measurements are reported of 20 OH/IR stars near the galactic center. This class of sources is known to have bright, intrinsically compact (less than or equal to 20 mas) maser components within their circumstellar shells. VLBA antennas and the VLA were used to perform a MKII spectral line VLBI experiment. The rapid drop in correlated flux with increasing baseline, especially for sources closest to the galactic center, is attributed to interstellar scattering. Angular diameters were measured for 13 of our sources. Lower limits were obtained for the remaining seven. With the data, together with additional data taken from the literature, the distribution was determined of interstellar scattering toward the galactic center. A region was found of pronounced scattering nearly centered on SgrA*. Two interpretations are considered for the enhanced scattering. One hypothesis is that the scattering is due to a clump of enhanced turbulence, such as those that lie along lines of sight to other known objects, that has no physical relationship to the galactic center. The other model considers the location of the enhanced scattering to arise in the galactic center itself. The physical implications of the models yield information on the nature of interstellar scattering
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