Astron

Abstract: In this paper we complete the set of diagnostic tools for synchrotron emitting sources presented by Del Zanna et al. (Astron. Astrophys. 453, 621, 2006) with the computation of inverse Compton radiation from the same relativistic particles. Moreover we investigate, for the first time, the gamma-ray emission properties of Pulsar Wind Nebulae in the light of the axisymmetric jet-torus scenario. The method consists in evolving the relativistic MHD equations and the maximum energy of the emitting particles. The particle energy distribution function is split in two components: the radio one connected to a relic population born at the outburst of the supernova and the other associated to the wind population continuously accelerated at the termination shock and emitting up to the gamma-ray band. We consider the general Klein-Nishina cross section and three different photon targets: the nebular synchrotron photons, far-infrared thermal ones and the cosmic microwave background. The overall synchrotron spectrum is fitted assuming an excess of injected particles and a steeper power law with respect to previous models. The TeV emission has the correct shape but is in excess of the data. This is due to the nebular magnetic field structure as obtained by the simulations. The jet-torus morphology is visible in high-resolution gamma-ray synthetic maps too. We present a preliminary exploration of time variability in the X and gamma-ray bands.

Abstract: The stability of a finite thickness, laminar cylindrical shell of electrons rotating azimuthally and enclosed in a coaxial waveguide is considered. The equilibrium rotation of the electrons is supported either by a radial electric field, an axial magnetic field, or a combination of both. The stability problem is formulated exactly as an eigenvalue problem, including all relativistic and electromagnetic effects as well as all effects of self and applied equilibrium fields. An approximate dispersion relation, valid for thin beams, is obtained analytically and the classical results for the ‘‘longitudinal’’ modes, i.e., the negative mass, cyclotron maser, and diocotron instabilities and for the ‘‘transverse’’ mode are recovered in appropriate limits. The dispersion relation is relatively simple and is valid for arbitrary values of the equilibrium electric and magnetic fields and for arbitrary beam energy. It therefore provides a ready comparison of the small signal properties of such devices as the Astron, gyrotron, orbitron, heliotron and the various cross field devices. It may also be of interest in accelerator and space physics applications. Some heretofore unnoticed effects on beam stability of equilibrium fields are reported; one such effect in particular leads to formulation of a simple, effective method either to maximize or eliminate altogether the longitudinal mode growth. Results from the dispersion relation compare favorably to results obtained from a numerical solution of the eigenvalue problem.

Abstract: The determination of a particle size distribution p(R) from small-angle scattering data is an example of a practical linear inverse problem for which there is no unique solution. It is shown (i) how the maximum entropy algorithm may be used to extract a unique solution which is the most uniform function compatible with the data set and is necessarily positive, and (ii) how the Backus–Gilbert [Geophys. J. R. Astron. Soc. (1968), 16, 169–205] method may be used to judge the significance of features in distributions derived from a data set of given statistical accuracy. As an illustration, void size distributions are presented for a sample of irradiated stainless steel which was successively annealed at increasing temperatures.

Abstract: The interstellar medium and solar wind is permeated by a magnetic field that renders magnetohydrodynamic turbulence anisotropic. In the classic work of Iroshnikov [Astron. Zh. 40, 742 (1963)] and Kraichnan [Phys. Fluids 8, 1385 (1965)], it is assumed that the turbulence is isotropic, and an inertial range energy spectrum that scales as k−3/2 is deduced based on the nonlinear interaction of Alfven wave packets. Much insight can be gained by analysis and high-resolution numerical simulations of such interactions. In the weak-turbulence limit in which three-wave interactions dominate, analytical and high-resolution numerical results based on random scattering of shear-Alfven waves propagating parallel to a large-scale magnetic field demonstrate an anisotropic energy spectrum that scales as k⊥−2. Even in the absence of a background magnetic field, when the energy spectrum is globally isotropic, anisotropy is found to develop with respect to the local magnetic field. The two-dimensional case is studied by mean...