arXiv subjects

H. Ardavan

Publications and source records attributed to H. Ardavan.

9 recordsLinked to original sources

arXiv2009

Comparison of multiwavelength observations of 9 broad-band pulsars with the spectrum of the emission from an extended current with a superluminally rotating distribution pattern

The observed spectra of 9 pulsars for which multiwavelength data are available from radio to $X$- or $γ$-ray bands (Crab, Vela, Geminga, B0656+14, B1055-52, B1509-58, B1706-44, B1929+10, and B1951+32) are compared with the spectrum of the radiation generated by an extended polarization current whose distribution pattern rotates faster than light {\it in vacuo}. It is shown that by inferring the values of two free parameters from observational data (values that are consistent with those of plasma frequency and electron cyclotron frequency in a conventional pulsar magnetosphere), and by adjusting the spectral indices of the power laws describing the source spectrum in various frequency bands, one can account {\em quantitatively} for the entire spectrum of each pulsar in terms of a single emission process. This emission process (a generalization of the synchrotron-Ćerenkov process to a volume-distributed source in vacuum) gives rise to an oscillatory radiation spectrum. Thus, the bell-shaped peaks of pulsar spectra in the ultraviolet or $X$-ray bands (the features that are normally interpreted as manifestations of thermal radiation) appear in the present model as higher-frequency maxima of the same oscillations that constitute the emission bands observed in the radio spectrum of the Crab pulsar. Likewise, the sudden steepening of the gradient of the spectrum by -1, which occurs around $10^{18}-10^{21}$ Hz, appears as a universal feature of the pulsar emission: a feature that reflects the transit of the position of the observer across the frequency-dependent Rayleigh distance. Inferred values of the free parameters of the present model suggest, moreover, that the lower the rotation frequency of a pulsar, the more weighted towards higher frequencies will be its observed spectral intensity.

astro-ph.GA↗

arXiv2008

Mechanism of generation of the emission bands in the dynamic spectrum of the Crab pulsar

We show that the proportionately spaced emission bands in the dynamic spectrum of the Crab pulsar (Hankins T. H. & Eilek J. A., 2007, ApJ, 670, 693) fit the oscillations of the square of a Bessel function whose argument exceeds its order. This function has already been encountered in the analysis of the emission from a polarization current with a superluminal distribution pattern: a current whose distribution pattern rotates (with an angular frequency $ω$) and oscillates (with a frequency $Ω>ω$ differing from an integral multiple of $ω$) at the same time (Ardavan H., Ardavan A. & Singleton J., 2003, J Opt Soc Am A, 20, 2137). Using the results of our earlier analysis, we find that the dependence on frequency of the spacing and width of the observed emission bands can be quantitatively accounted for by an appropriate choice of the value of the single free parameter $Ω/ω$. In addition, the value of this parameter, thus implied by Hankins & Eilek's data, places the last peak in the amplitude of the oscillating Bessel function in question at a frequency ($\simΩ^3/ω^2$) that agrees with the position of the observed ultraviolet peak in the spectrum of the Crab pulsar. We also show how the suppression of the emission bands by the interference of the contributions from differring polarizations can account for the differences in the time and frequency signatures of the interpulse and the main pulse in the Crab pulsar. Finally, we put the emission bands in the context of the observed continuum spectrum of the Crab pulsar by fitting this broadband spectrum (over 16 orders of magnitude of frequency) with that generated by an electric current with a superluminally rotating distribution pattern.

astro-ph↗

arXiv2007

Occurrence of concurrent `orthogonal' polarization modes in the Lienard-Wichert field of a rotating superluminal source

We evaluate the Lienard-Wiechert field of a rotating superluminal point source numerically and show that this radiation field has the following intrinsic characteristics. (i) It is sharply focused along a narrow, rigidly rotating spiral-shaped beam that embodies the cusp of the envelope of the emitted wave fronts. (ii) It consists of either one or three concurrent polarization modes (depending on the relative positions of the observer and the cusp) that constitute contributions to the field from differing retarded times. (iii) Two of the modes are comparable in strength at both edges of the signal and dominate over the third everywhere except in the middle of the pulse. (iv) The position angle of the total field swings across the beam by as much as 180$^\circ$. (v) The position angles of its two dominant modes remain approximately orthogonal throughout their excursion across the beam. Given the fundamental nature of the Lienard-Wiechert field, the coincidence of these characteristics with those of the radio emission that is received from pulsars is striking.

astro-ph↗

arXiv2006

Comment on "Radial dependence of radiation from a bounded source" by Kirk T. McDonald

The purpose of this note is to point out that McDonald's criticism of our work \cite{r1} is based on a circular argument. In order to show that the field of a bounded source falls off as $f(θ,ϕ)/r$ in the far zone, McDonald uses a Huygens-Kirchhoff diffraction integral whose derivation (from Maxwell's equations) already entails assuming a fall-off of this form for the field at infinity \cite{r2}. ($r$, $θ$ and $ϕ$ are the spherical polar coordinates centred on a point within the source, and $f$ is a factor independent of $r$.

physics.optics↗

arXiv2004

Experimental demonstration of a new radiation mechanism: emission by an oscillating, accelerated, superluminal polarization current

We describe the experimental implementation of a superluminal ({\it i.e.} faster than light {\it in vacuo}) polarization current distribution that both oscillates and undergoes centripetal acceleration. Theoretical treatments lead one to expect that the radiation emitted from each volume element of such a polarization current will comprise a Čerenkov-like envelope with two sheets that meet along a cusp. The emission from the experimental machine is in good agreement with these expectations, the combined effect of the volume elements leading to tightly-defined beams of a well-defined geometry, determined by the source speed and trajectory. In addition, over a restricted range of angles, we detect the presence of cusps in the emitted radiation. These are due to the detection over a short time period (in the laboratory frame) of radiation emitted over a considerably longer period of source time. Consequently, the intensity of the radiation at these angles was observed to decline more slowly with increasing distance from the source than would the emission from a conventional antenna. The angular distribution of the emitted radiation and the properties associated with the cusps are in good {\it quantitative} agreement with theoretical models of superluminal sources once the effect of reflections from the earth's surface are taken into account.

physics.optics↗

arXiv2003

The spectral and polarization characteristics of the nonspherically decaying radiation generated by polarization currents with superluminally rotating distribution patterns

We present a theoretical study of the emission from a superluminal polarization current whose distribution pattern rotates (with an angular frequency $ω$) and oscillates (with a frequency $Ω$) at the same time, and which comprises both poloidal and toroidal components. This type of polarization current is found in recent practical machines designed to investigate superluminal emission. We find that the superluminal motion of the distribution pattern of the emitting current generates localized electromagnetic waves that do not decay spherically, i.e. that do not have an intensity diminishing like ${R_P}^{-2}$ with the distance $R_P$ from their source. The nonspherical decay of the focused wave packets that are emitted by the polarization currents does not contravene conservation of energy: the constructive interference of the constituent waves of such propagating caustics takes place within different solid angles on spheres of different radii ($R_P$) centred on the source. For a polarization current whose longitudinal distribution (over an azimuthal interval of length $2π$) consists of $m$ cycles of a sinusoidal wave train, the nonspherically decaying part of the emitted radiation contains the frequencies $Ω\pm mω$; i.e. it contains {\it only} the frequencies involved in the creation and implementation of the source. This is in contrast to recent studies of the spherically decaying emission, which was shown to contain much higher frequencies. The polarization of the emitted radiation is found to be linear for most configurations of the source.

physics.optics↗

arXiv2002

The frequency spectrum of focused broadband pulses of electromagnetic radiation generated by polarization currents with superluminally rotating distribution patterns

We investigate the spectral features of the emission from a superluminal polarization current whose distribution pattern rotates with an angular frequency $ω$ and oscillates with an incommensurate frequency $Ω>ω$. This type of polarization current is found in recent practical machines designed to investigate superluminal emission. Although all of the processes involved are linear, we find that the broadband emission contains frequencies that are higher than $Ω$ by a factor of the order of $(Ω/ω)^2$. This generation of frequencies {\it not} required for the creation of the source stems from mathematically rigorous consequences of the familiar classical expression for the retarded potential. The results suggest practical applications for superluminal polarization currents as broad-band radiofrequency and infrared sources.

physics.optics↗

arXiv1995

Violation of the inverse square law by the emissions of supersonically and superluminally moving volume sources

The generally familiar notion that the conservation of energy requires the intensity of the radiation generated by a localized finite-duration source to decay like the inverse square of the distance from the source is not necessarily true. In this paper, we identify physically tenable sources of acoustic and electromagnetic radiations the amplitudes of whose emissions to particular distant zones decay cylindrically (like {R_P}^{-{1\over2}}) rather than spherically (like {R_P}^{-1}) as R_P tends to infinity (R_P denotes the distance of the observer from the source). These sources have moving distribution patterns which are in general three-dimensional and which propagate faster than the emitted waves. Their emission is characterized by a waveform of constant duration that consists of a continuous assemblage of cylindrically decaying subpulses. Each subpulse embodies a propagating caustic and is narrower the further away it is observed from the source. The change in the lifetime of the subpulses with range (\sim {R_P}^{-1}) is such that their energy---but not their intensity---follows the inverse square law and the Rayleigh distance associated with them is of the same order of magnitude as their distance from the source (R_P) for all values of this distance.\par We present our work in the context of the literature on the non-diffracting wave packets known as acoustic or electromagnetic missiles, and point out how these missiles allow the existing body of data on the emissions from supersonic jets and propellers and from pulsars to be seen in a different light. A supersonically convected aeroacoustic source of volumetric scale L^3 and lifetime T radiates conventional Mach waves whose mean square pressure-fluctuations level scales as ρ^2 U^4cTL /{R_P}^2 (ρand U are

astro-ph↗

arXiv1995

Splitting of the Alfven surface in a relativistic pulsar wind

In a recent paper, Li and Melrose have claimed that the splitting - due to relativistic effects - of the Alfven surface in an axisymmetric pulsar wind does not occur. Here we refute this claim by showing that, unless the solution that describes the flow along each open magnetic field line passes through the pure Alfvenic point (which is one of the manifestations of the splitting of the Alfvenic point), it would not be physically viable both at the surface of the star and at infinity.

astro-ph↗