2,438 research outputs found
Pulse-to-pulse intensity modulation and drifting subpulses in recycled pulsars
We report the detection of pulse-to-pulse periodic intensity modulations, in
observations of recycled pulsars. Even though the detection of individual
pulses was generally not possible due to their low flux density and short
duration, through the accumulation of statistics over sequences of 10^5--10^6
pulses we were able to determine the presence and properties of the
pulse-to-pulse intensity variations of six pulsars. In most cases we found that
the modulation included a weak, broadly quasi-periodic component. For two
pulsars the sensitivity was high enough to ascertain that the modulation phase
apparently varies systematically across the profile, indicating that the
modulation appears as drifting subpulses. We detected brighter than average
individual pulses in several pulsars, with energies up to 2--7 times higher
than the mean, similar to results from normal pulsars. We were sensitive to
giant pulses of a rate of occurrence equal to (and in many instances much lower
than) that of PSR B1937+21 at 1400 MHz (~30 times lower than at 430 MHz), but
none were detected, indicating that the phenomenon is rare in recycled pulsars.Comment: 15 pages, 17 figures, accepted to A&
Astronomy using basic Mark 2 very long baseline interferometry
Two experiments were performed in April and September 1976 to determine precise positions of radio sources using conventional Mark 2 VLBI techniques. Four stations in the continental United States observed at a wavelength of 18 cm. The recording bandwidth was 2 MHz. The preliminary results using analyses of fringe rate and delay are discussed and the source positions compared with the results of other measurements
Radio-wave propagation through a medium containing electron-density fluctuations described by an anisotropic Goldreich-Sridhar spectrum
We study the propagation of radio waves through a medium possessing density
fluctuations that are elongated along the ambient magnetic field and described
by an anisotropic Goldreich-Sridhar power spectrum. We derive general formulas
for the wave phase structure function, visibility, angular broadening,
diffraction-pattern length scales, and scintillation time scale for arbitrary
distributions of turbulence along the line of sight, and specialize these
formulas to idealized cases.Comment: 25 pages, 3 figures, submitted to Ap
Ellipticity and Deviations from Orthogonality in the Polarization Modes of PSR B0329+54
We report on an analysis of the polarization of single pulses of PSR B0329+54
at 328 MHz. We find that the distribution of polarization orientations in the
central component diverges strongly from the standard picture of orthogonal
polarization modes (OPMs), making a remarkable partial annulus on the Poincare
sphere. A second, tightly clustered region of density appears in the opposite
hemisphere, at a point antipodal to the centre of the annulus. We argue that
this can be understood in terms of birefringent alterations in the relative
phase of two elliptically polarized propagation modes in the pulsar
magnetosphere (i.e. generalised Faraday rotation). The ellipticity of the modes
implies a significant charge density in the plasma, while the presence of both
senses of circular polarization, and the fact that only one mode shows the
effect, supports the view that refracted ordinary-mode rays are involved in the
production of the annulus. At other pulse longitudes the polarization
(including the circular component) is broadly consistent with an origin in
elliptical OPMs, shown here quantitatively for the first time, however
considerable non-orthogonal contributions serve to broaden the orientation
distribution in an isotropic manner.Comment: 13 pages, 5 figures, to appear in A&
Low Speed Estimation of Sensorless DTC Induction Motor Drive Using MRAS with Neuro Fuzzy Adaptive Controller
This paper presents a closed loop Model Reference Adaptive system (MRAS) observer with artificial intelligent Nuero fuzzy controller (NFC) as the adaptation technique to mitigate the low speed estimation issues and to improvise the performance of the Sensorless Direct Torque Controlled (DTC) Induction Motor Drives (IMD). Rotor flux MRAS and reactive power MRAS with NFC is explored and detailed analysis is carried out for low speed estimation. Comparative analysis between rotor flux MRAS and reactive power MRAS with PI as well as NFC as adaptive controller is performed and results are presented in this paper. The comparative analysis among these four speed estimation methods shows that reactive power MRAS with NFC as adaptation mechanism shows reduced speed estimation error and actual speed error at steady state operating conditions when the drive is subjected to low speed operation. Simulation carried out using MATLAB-Simulink software to validate the performance of the drive especially at low speeds with rated and variable load conditions
Classifying hyperspectral airborne imagery for vegetation survey along coastlines
This paper studies the potential of airborne hyperspectral imagery for classifying vegetation along the Belgian coastlines. Here, the aim is to build vegetation maps using automatic classification. Besides a general linear multiclass classifier (Linear Discriminant Analysis), several strategies for combining binary classifiers are proposed: one based on a hierarchical decision tree, one based on the Hamming distance between the codewords obtained by binary classifiers and one based on the coupling of posterior probabilities. In addition, a new procedure is proposed for spatial classification smoothing. This procedure takes into account spatial information by letting the decision for classification of a pixel depend on the classification probabilities of neighboring pixels. This is shown to render smoother classification images
Can billiard eigenstates be approximated by superpositions of plane waves?
The plane wave decomposition method (PWDM) is one of the most popular
strategies for numerical solution of the quantum billiard problem. The method
is based on the assumption that each eigenstate in a billiard can be
approximated by a superposition of plane waves at a given energy. By the
classical results on the theory of differential operators this can indeed be
justified for billiards in convex domains. On the contrary, in the present work
we demonstrate that eigenstates of non-convex billiards, in general, cannot be
approximated by any solution of the Helmholtz equation regular everywhere in
(in particular, by linear combinations of a finite number of plane waves
having the same energy). From this we infer that PWDM cannot be applied to
billiards in non-convex domains. Furthermore, it follows from our results that
unlike the properties of integrable billiards, where each eigenstate can be
extended into the billiard exterior as a regular solution of the Helmholtz
equation, the eigenstates of non-convex billiards, in general, do not admit
such an extension.Comment: 23 pages, 5 figure
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