Emission Cone Research Articles

Pulse Width Measurements for Pulsars

There are a few methods of measuring pulse widths. Among the most often used pulse widths there are W pp (the distance between two peak value points), W 50 (the width at the place of 50% of the highest peak intensity) and W10 (the width at the place of 10% of the highest peak intensity). In addition, there are also the width W c50 at the place of 50% of their respective intensities and the width Wc10 at the place of 10% of the respective intensities of the two outermost components. The data of the pulse profiles of the sample of 16 double-cone peak pulsars and 7 core-cone triple peak pulsars at 1.4 GHz are utilized to measure 5 kinds of pulse widths. The pulsars with the data of the magnetic inclination α and impact angle β are employed to derive the corresponding radiation beam radius ρ, and the relation between ρ and the pulsar period P, ρ ∝ P −0.5, is verified. Via a comparison between the qualities of the relation between ρ and P derived from the 5 kinds of pulse widths it is found that the width between the outermost components is better than any one of the first 3 kinds of widths, of which the correlative relation obtained from the W c50 width is the best. From this it is considered that W c50 is the measured value that can fully reflect the emission beam width. The symmetry of the pulse profile is also discussed and it is found that from the comparison with the preceding cone emission component the following cone emission component is much closer to the core emission component, and the widths of the two cone emission components are basically the same according to statistics.

The geometry of radio pulsar magnetospheres

Data on the profiles and polarization of the 10- and 20-cm emission of radio pulsars are used to calculate the angle β between the rotational axis of the neutron star and its magnetic moment. It is shown that, for these calculations, it is sufficient to use catalog values of the pulse width at the 10% level W10, since the broadening of the observed pulses due to the transition to the full width W0 and narrowing of the pulses associated with the emission of radiation along tangents to the field lines approximately cancel each other out. The angles β1 are calculated for 283 pulsars at 20 cm and 132 pulsars at 10 cm, assuming that the line of sight passes through the center of the emission cone. The mean values of these angles are small and similar for the two wavelengths (〈β1〉 = 18° at λ = 10 cm and 〈β1〉 = 14° at λ = 20 cm). The angle β2 is estimated for several dozen pulsars for the case when the orientation of the angle to the line of sight is arbitrary. The mean value of β2 at 10 cm is found to be 〈β2〉 = 33.9° if the maximum derivative of the polarization position angle C is positive and 〈β2〉 = 52.1° ifC 0 and 〈β2〉 = 54.1° ifC 0 and 〈β2〉 = 49.1° if C < 0). The mean 〈β2〉 for the entire set of data can be taken to be 43.5°. The period dependence of the pulse width W(P) √ P−0.25 differs from the relation that is usually used in the polar-cap model, W(P) √ P−0.5. This difference could be associated with the rate of development of plasma instabilities near the surface of the neutron star (in the region where high-frequency radiation is generated). The role of the quadrupole component of the magnetic field is not important here. There is no dependence of the angle β on the pulsar age (z distance, luminosity L, or characteristic age τ = P/(2dP/dt)) for the studied sample.

Controlled leaky wave radiation from anisotropic epsilon near zero metamaterials

We investigate the emission of electromagnetic waves from biaxial subwavelength metamaterials. For tunable anisotropic structures that exhibit a vanishing dielectric response along a given axis, we find remarkable variation in the launch angles of energy associated with the emission of leaky wave radiation. We write closed form expressions for the energy transport velocity and corresponding radiation angle $\phi$, defining the cone of radiation emission, both as a functions of frequency, and material and geometrical parameters. Full wave simulations exemplify the broad range of directivity that can be achieved in these structures.

Formation of “lightnings” in a neutron star magnetosphere and the nature of RRATs

The connection between the radio emission from "lightnings" produced by the absorption of high-energy photons from the cosmic gamma-ray background in a neutron star magnetosphere and radio bursts from rotating radio transients (RRATs) is investigated. The lightning length reaches 1000 km; the lightning radius is 100 m and is comparable to the polar cap radius. If a closed magnetosphere is filled with a dense plasma, then lightnings are efficiently formed only in the region of open magnetic field lines. For the radio emission from a separate lightning to be observed, the polar cap of the neutron star must be directed toward the observer and, at the same time, the lightning must be formed. The maximum burst rate is related to the time of the plasma outflow from the polar cap region. The typical interval between two consecutive bursts is ~100 s. The width of a single radio burst can be determined both by the width of the emission cone formed by the lightning emitting regions at some height above the neutron star surface and by a finite lightning lifetime. The width of the phase distribution for radio bursts from RRATs, along with the integrated pulse width, is determined by the width of the bundle of open magnetic field lines at the formation height of the radio emission. The results obtained are consistent with the currently available data and are indicative of a close connection between RRATs, intermittent pulsars, and extreme nullers.

Fundamental limit of nanophotonic light trapping in solar cells

Establishing the fundamental limit of nanophotonic light-trapping schemes is of paramount importance and is becoming increasingly urgent for current solar cell research. The standard theory of light trapping demonstrated that absorption enhancement in a medium cannot exceed a factor of 4n(2)/sin(2)θ, where n is the refractive index of the active layer, and θ is the angle of the emission cone in the medium surrounding the cell. This theory, however, is not applicable in the nanophotonic regime. Here we develop a statistical temporal coupled-mode theory of light trapping based on a rigorous electromagnetic approach. Our theory reveals that the conventional limit can be substantially surpassed when optical modes exhibit deep-subwavelength-scale field confinement, opening new avenues for highly efficient next-generation solar cells.

LOCALIZATION OF A TYPE III RADIO BURST OBSERVED BY THESTEREOSPACECRAFT

Ray tracing calculations show that (1) emissions from a localized source escape as direct and reflected waves along different paths, (2) the reflected waves experience higher attenuation and group delay because they travel longer path lengths in regions of reduced refractive index, and (3) widely separated spacecraft "A" and "B" can detect the direct as well as reflected emissions escaping along different directions. It is proposed that the source of a radio burst observed by twin spacecraft "A" and "B" can be localized if at a given frequency the emission at one of them is identified as the direct emission and is identified at the other as the reflected emission by comparing the observed time delays ΔT, as well as intensity ratios IB/IA with the corresponding values of the direct and reflected emissions obtained for a given coronal model. A type III event observed by the STEREO spacecraft "A" and "B" shows that its characteristics are consistent with direct and reflected emissions by being less intense and delayed at "A" in comparison to that at "B." By applying the proposed technique to this event, the location of its source is found to lie between the turning point of the ray and the harmonic layer corresponding to fpe = f/2, where f and fpe are the frequency of the emission and the electron plasma frequency, respectively. The comparisons of the widths of the fundamental and harmonic emission cones with the angular separation of spacecraft "A" and "B" indicate that the mode of the observed emission is probably the harmonic.

Planar photonic crystal cavities with far-field optimization for high coupling efficiency and quality factor

Different types of planar photonic crystal cavities aimed at optimizing the far-field emission pattern are designed and experimentally assessed by resonant scattering measurements. We systematically investigate the interplay between achieving the highest possible quality (Q) factor and maximizing the in- and out-coupling efficiency into a narrow emission cone. Cavities operate at telecommunications wavelengths, i.e. around approximately 1.55 microm, and are realized in silicon membranes. A strong modification of the far-field emission pattern, and therefore a substantial increase of the coupling efficiency in the vertical direction, is obtained by properly modifying the holes around L3, L5 and L7 type PhC cavities, as we predict theoretically and show experimentally. An optimal compromise yielding simultaneously a high Q-factor and a large coupling to the fundamental cavity mode is found for a L7-type cavity with a measured Q congruent with 62000, whose resonant scattering efficiency is improved by about two orders of magnitude with respect to the unmodified structure. These results are especially useful for prospective applications in light emitting devices, such as nano-lasers or single-photon sources, in which vertical in- and out-coupling of the electromagnetic field is necessarily required.

Pulsar magnetic alignment and the pulsewidth-age relation

Using pulsewidth data for 872 isolated radio pulsars, we test the hypothesis that pulsars evolve through a progressive narrowing of the emission cone combined with progressive alignment of the spin and magnetic axes. The new data provide strong evidence for the alignment over a time-scale of about 1 Myr with a log standard deviation of around 0.8 across the observed population. This time-scale is shorter than the time-scale of about 10 Myr found by previous authors, but the log standard deviation is larger. The results are inconsistent with models based on magnetic field decay alone or monotonic counter-alignment to orthogonal rotation. The best fits are obtained for a braking index parameter, nγ≈ 2.3, consistent with the mean of the six measured values, but based on a much larger sample of young pulsars. The least-squares fitted models are used to predict the mean inclination angle between the spin and magnetic axes as a function of log characteristic age. Comparing these predictions to existing estimates, it is found that the model in which pulsars are born with a random angle of inclination gives the best fit to the data. Plots of the mean beaming fraction as a function of characteristic age are presented using the best-fitting model parameters.

Flipping a photonic shock wave

Cerenkov radiation with the emission cone reversed has been observed in a metamaterial with negative refractive index.

THE DUST ENVIRONMENT OF COMET 29P/SCHWASSMANN-WACHMANN 1 FROM DUST TAIL MODELING OF 2004 NEAR-PERIHELION OBSERVATIONS

A Monte Carlo inverse dust tail modeling of ground-based images of comet 29P/Schwasssmann-Wachmann 1 has been performed. The images of the comet were acquired on several nights in 2004 July, a few days after the 2004 perihelion passage. The analysis takes into account the rotation properties of the comet, incorporating dust ejection from active areas on the nucleus surface. We demonstrate that these models provide a significant improvement over models with a fixed sunward hemispherical particle emission cone, owing to the observed coma asymmetry, giving an excellent fit to the observed intensity isophote fields. The rotation parameters, defined by the argument of the subsolar meridian at perihelion, Φ, and the obliquity, I, are found to be compatible with those derived by Sekanina from morphological studies (Φ = 279° and I = 100°). We found that if dust emission is assumed to be produced by a single active area driven by insolation, this must then be located on the southern hemisphere near –35° latitude. We have devised a method to impose Afρ(t) constraints the overdetermined system of equations leading to the solution of the dust mass loss rates and size distribution function. When those constraints are applied, the time-averaged particle size distribution function was found to be characterized by a power law of index in the range –3.7 to –3.3, and a dust loss mass rate approximately in the nominal range of 300-900 kg s–1, depending on the different model approaches, and for an albedo time the phase function of 0.1, confirming the fact that this comet is perhaps the most active source of interplanetary dust, providing some 3%-10% of the mass required to replenish the losses of the interplanetary dust cloud if it is in a steady state.

Characterizing the gravitational wave signature from cosmic string cusps

Cosmic strings are predicted to form kinks and cusps that travel along the string at close to the speed of light. These disturbances are radiated away as highly beamed gravitational waves that produce a burst like pulse as the cone of emission sweeps past an observer. Gravitational wave detectors such as the Laser Interferometer Space Antenna (LISA) and the Laser Interferometer Gravitational wave Observatory (LIGO) will be capable of detecting these bursts for a wide class of string models. Such a detection would illuminate the fields of string theory, cosmology, and relativity. Here we develop template based Markov Chain Monte Carlo (MCMC) techniques that can efficiently detect and characterize the signals from cosmic string cusps. We estimate how well the signal parameters can be recovered by the advanced LIGO-Virgo network and the LISA detector using a combination of MCMC and Fisher matrix techniques. We also consider joint detections by the ground and space based instruments. We show that a parallel tempered MCMC approach can detect and characterize the signals from cosmic string cusps, and we demonstrate the utility of this approach on simulated data from the third round of Mock LISA Data Challenges (MLDCs).

TeV variability in blazars: how fast can it be?

Abstract Recent Cerenkov observations of the two BL Lac objects PKS 2155–304 and Mkn 501 revealed TeV flux variability by a factor of ∼2 in just 3–5 min. Even accounting for the effects of relativistic beaming, such short time-scales are challenging simple and conventional emitting models, and call for alternative ideas. We explore the possibility that extremely fast variable emission might be produced by the particles streaming at ultra-relativistic speeds along magnetic field lines and inverse-Compton scattering any radiation field already present. This would produce extremely collimated beams of TeV photons. While the probability for the line of sight to be within such a narrow cone of emission would be negligibly small, one would expect that the process is not confined to a single site, but can take place in many very localized regions, along almost straight magnetic lines. A possible astrophysical setting realizing these conditions is magneto-centrifugal acceleration of beams of particles. In this scenario, the variability time-scale would not be related to the physical dimension of the emitting volume, but might be determined by either the typical duration of the process responsible for the production of these high-energy particle beams or by the coherence length of the magnetic field. It is predicted that even faster TeV variability — with no X-ray counterpart — should be observed by the foreseen more sensitive Cerenkov telescopes.

A simple solution for an intense terahertz emitter

Terahertz (THz, in the far IR region) imaging and spectroscopy offers the ability to discover chemical composition noninvasively. Applications include medical imaging, security screening, and pharmaceutical quality control. Semiconductors can produce pulsed THz radiation when their surface is illuminated by a sub-picosecond laser pulse, rather than continuous wave sources, such as a quantum cascade laser. Yet despite several technical advances, the output power is low. Thus the development of higher power semiconductor THz emitters is of considerable practical significance.1 THz generation occurs when a semiconductor is illuminated by an ultrafast laser pulse with photon energy greater than the semiconductor bandgap. A large number of electron and hole pairs are created and accelerated in opposite directions by the electric field. The resulting charge separation forms a dipole that emits a coherent THz pulse. THz radiation generated within a material of high refractive index n has an extraction problem: at the semiconductor surface, all rays outside an ‘emission cone’ of half-angle α = sin−1 1 n to the surface normal (shown as a grey cone in Figure 1) suffer total internal reflection and do not escape from the device. The fraction of power extracted depends on the orientation of the dipole axis relative to the surface. Several methods exist to increase the extraction efficiency, such as applying an external magnetic field,2 using a coupling prism,3 or fabrication of low-dimensional nanostructures.4 For example, under a magnetic field the Lorentz force—the force on a point charge due to electromagnetic fields—can rotate the orientation of the dipole to coincide with the emission cone as shown in Figure 1(a). For the extreme case of a dipole parallel to the surface, the emitted power would increase by more than a factor of 20. But these methods require cumbersome equipment, such as a strong magnet, or special sample growth techniques. We simply changed the growth orientation of the Figure 1. (a) By applying an external magnetic field (B), the THz dipole can be rotated and the radiation pattern can be overlapped with the emission cone to enhance the free-space radiation. (b) The wurtzite (hexagonal crystal) structure of indium nitride (InN). The InN film is grown along the a-axis and the in-plane electric field is formed on the a-plane.

Pecularities in produced-particle emission in 208Pb+ Ag(Br) interactions at 158 A GeV/c

The angular structures of particles produced in 208Pb-induced collisions with Ag(Br) nuclei in an emulsion detector at 158 A GeV/c have been investigated. Nonstatistical ringlike substructures in the azimuthal plane of the collision have been found and their parameters have been determined. An indication of the formation of ringlike substructures from two symmetrical emission cones, one in the forward and the other in the backward direction in the center-of-mass system, has been obtained. The ringlike substructures’ parameters have been determined. The experimental results are in an agreement with I.M. Dremin’s idea that the mechanism of the ringlike substructures’ formation in nuclear collisions is similar to that of Cherenkov electromagnetic radiation.

Time-resolved measurements of the angular distribution of the Ni-like Pd soft-x-ray laser emission

The temporal history of the x-ray laser output has been measured in dependence of the emission angle of the x-ray laser with respect to the target plane. The results show both the exit time (the time at which the x-ray laser pulse is leaving the plasma) and the pulse duration to depend on the angle of emission. Simulations of the gain in Ni-like Pd combined with ray tracing through the plasma were used to model the dependence of the time of emission on the deflection angle of the individual rays within the laser emission cone. The simulations are in reasonable agreement with the measurements.

Cone emission induced by femtosecond excitation in rubidium vapor

We studied the emission from dense rubidium vapor under 100-fs pulsed laser excitation in the 730--820 nm range. Self-focusing along with a conical emission (CE) was observed for the excitation in the blue wing of $D2$ atomic resonance. Self-defocusing occurred for the red-wing excitation. This is in accordance with the theory of the intensity dependent nonlinear refraction index. In the case of the CE observation, our experimental results are attributed to the self-phase modulation of the laser beam interacting with the near-resonant atomic medium.

High‐Altitude Emission from Pulsar Slot Gaps: The Crab Pulsar

We present results of a 3D model of optical to gamma-ray emission from the slot gap accelerator of a rotation-powered pulsar. Primary electrons accelerating to high-altitudes in the unscreened electric field of the slot gap reach radiation-reaction limited Lorentz factors of 2 x 10^7, while electron-positron pairs from lower-altitude cascades flow along field lines interior to the slot gap. The curvature, synchrotron and inverse Compton radiation of both primary electrons and pairs produce a broad spectrum of emission from infra-red to GeV energies. Both primaries and pairs undergo cyclotron resonant absorption of radio photons, allowing them to maintain significant pitch angles. Synchrotron radiation from pairs with a power-law energy spectrum with Lorentz factors 10^2 - 10^5, dominate the spectrum up to 10 MeV. Synchrotron and curvature radiation of primaries dominates from 10 MeV up to a few GeV. We examine the energy-dependent pulse profiles and phase-resolved spectra for parameters of the Crab pulsar as a function of magnetic inclination and viewing angle, comparing to broad-band data. In most cases, the pulse profiles are dominated by caustics on trailing field lines. We also explore the relation of the high-energy and the radio profiles, as well as the possibility of caustic formation in the radio cone emission. We find that the Crab pulsar profiles and spectrum can be reasonably well reproduced by a model with viewing angle 45 degrees and inclination angle 100 or 80 degrees. This model predicts that the slot gap emission below 200 MeV will exhibit correlations in time and phase with the radio emission.

Population Synthesis of Radio and γ‐Ray Millisecond Pulsars from the Galactic Disk

We present results of a population synthesis of millisecond pulsars from the Galactic disk. Excluding globular clusters, we model the spatial distribution of millisecond pulsars by assuming their birth in the Galactic disk with a random kick velocity and evolve them to the present within the Galactic potential. We assume that normal and millisecond pulsars are standard candles described with a common radio luminosity model that invokes a new relationship between radio core and cone emission suggested by recent studies. In modeling the radio emission beams, we explore the relativistic effects of time delay, aberration, and sweep-back of the open field lines. While these effects are essential to understanding pulse profiles, the phase-averaged flux is adequately described without a relativistic model. We use a polar cap acceleration model for the ?-ray emission. We present the preliminary results of our recent study and the implications for observing millisecond pulsars with GLAST and AGILE.

Effects of molecular orientation on surface-plasmon-coupled emission patterns

We observed azimuthal distributions of surface-plasmon-coupled emission from thin layers of randomly oriented and oriented sexithiophene molecules deposited on 50-nm-thick Ag films. For the randomly oriented layer, we observed a uniform and isotropic cone of emission, showing that the surface plasmons are excited isotropically in all directions on the Ag surface. The emission pattern for the oriented layer exhibited a much stronger intensity along the direction of molecular orientation, showing that surface plasmons propagating parallel to the molecular orientation are excited most effectively. These results demonstrate that surface-plasmon-coupled emission provides a new simple approach to the determination of molecular orientation in thin films.

Models for the Comet Dynamical Environment

An outgassing jet model is presented in support of spacecraft navigation for future missions to comets. The outgassing jet is modeled as an emission cone while the comet is modeled as a uniform density triaxial ellipsoid. The comet's motion about the sun is included in the model. The model is used to explore the effects on a spacecraft passing through an outgassing jet field. The outgassing jet model is also used for simulation and estimation of the physical outgassing properties of jets at and near the surface of a comet. Methods for estimating the locations and sizes of multiple outgassing jets are presented.