Observational constraints on solar wind acceleration mechanisms

A complete theoretical understanding of the acceleration of the solar wind must account for at least three types of solar wind flow: high-speed streams associated with coronal holes, low-speed boundary layer flows associated with sector boundaries, and both high- and low-speed flows associated with impulsive ejections from the Sun. The properties of each type of flow are summarized

Particle acceleration mechanisms

High-energy particle acceleration is observed to proceed in a diverse variety of astrophysical sites ranging from the terrestrial aurorae to the most distant quasars. Particle acceleration is a fairly common channel for the release of large-scale kinetic, rotational, and magnetic energy. Physical mechanisms include electrostatic acceleration, stochastic processes, and diffusive shock energization. Cosmic-ray energy spectra have shapes which reflect escape, collisional, and radiative losses. The overall acceleration efficiency is controlled by the low-energy particle injection which may, in turn, feed back into the energization. Recent observational developments, which illustrate these general principles and raise fresh questions, are briefly summarized

Particle acceleration mechanisms

We review the possible mechanisms for production of non-thermal electrons which are responsible for non-thermal radiation in clusters of galaxies. Our primary focus is on non-thermal Bremsstrahlung and inverse Compton scattering, that produce hard X-ray emission. We briefly review acceleration mechanisms and point out that in most astrophysical situations, and in particular for the intracluster medium, shocks, turbulence and plasma waves play a crucial role. We consider two scenarios for production of non-thermal radiation. The first is hard X-ray emission due to non-thermal Bremsstrahlung by nonrelativistic particles. Non-thermal tails are produced by accelerating electrons from the background plasma with an initial Maxwellian distribution. However, these tails are accompanied by significant heating and they are present for a short time of <10^6 yr, which is also the time that the tail will be thermalised. Such non-thermal tails, even if possible, can only explain the hard X-ray but not the radio emission which needs GeV or higher energy electrons. For these and for production of hard X-rays by the inverse Compton model, we need the second scenario where there is injection and subsequent acceleration of relativistic electrons. It is shown that a steady state situation, for example arising from secondary electrons produced from cosmic ray proton scattering by background protons, will most likely lead to flatter than required electron spectra or it requires a short escape time of the electrons from the cluster. An episodic injection of relativistic electrons, presumably from galaxies or AGN, and/or episodic generation of turbulence and shocks by mergers can result in an electron spectrum consistent with observations but for only a short period of less than one billion years.Comment: 22 pages, 5 figures, accepted for publication in Space Science Reviews, special issue "Clusters of galaxies: beyond the thermal view", Editor J.S. Kaastra, Chapter 11; work done by an international team at the International Space Science Institute (ISSI), Bern, organised by J.S. Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke

Astrophysical sources and acceleration mechanisms

Multi-messenger astronomy provides for the observation of the same astronomical event with different kind of telescopes at the same time: optical observations, X-rays, gamma-ray bursts, neutrinos and, most recently, gravitational waves are just few examples of the several points of view from which an astronomical event can be observed and analyzed. Cosmic rays play an important role in multi-messenger astronomy and, for this reason, it is important to deepen the study of their sources and to understand the mechanisms behind their acceleration in astronomical environments

Modification of cosmic-ray energy spectra by stochastic acceleration

Context: Typical space plasmas contain spatially and temporally variable turbulent electromagnetic fields. Understanding the transport of energetic particles and the acceleration mechanisms for charged particles is an important goal of today's astroparticle physics. Aims: To understand the acceleration mechanisms at the particle source, subsequent effects have to be known. Therefore, the modification of a particle energy distribution, due to stochastic acceleration, needs to be investigated. Methods: The diffusion in momentum space was investigated by using both a Monte-Carlo simulation code and by analytically solving the momentum-diffusion equation. For simplicity, the turbulence was assumed to consist of one-dimensional Alfven waves. Results: Using both methods, it is shown that, on average, all particles with velocities comparable to the Alfven speeds are accelerated. This influences the energy distribution by significantly increasing the energy spectral index. Conclusions: Because of electromagnetic turbulence, a particle energy spectrum measured at Earth can drastically deviate from its initial spectrum. However, for particles with velocities significantly above the Alfven speed, the effect becomes negligible.Comment: 10 pages, 6 figures, accepted for publication in Astronomy & Astrophysic

High-energy emission from off-axis relativistic jets

We analyze how the spectrum of synchrotron and inverse Compton radiation from a narrow relativistic jet changes with the observation angle. It is shown that diversity of acceleration mechanisms (in particular, taking the converter mechanism (Derishev et al. 2003) into account) allows for numerous modifications of the observed spectrum. In general, the off-axis emission in GeV-TeV energy range appears to be brighter, has a much harder spectrum and a much higher cut-off frequency compared to the values derived from Doppler boosting considerations alone. The magnitude of these effects depends on the details of particle acceleration mechanisms, what can be used to discriminate between different models. One of the implications is the possibility to explain high-latitude unidentified EGRET sources as off-axis but otherwise typical relativistic-jet sources, such as blazars. We also discuss the broadening of beam pattern in application to bright transient jet sources, such as Gamma-Ray Bursts.Comment: 6 pages, Proceedings of the International Symposium "High Energy Gamma-Ray Astronomy", 26-30 July 2004, Heidelberg, German

Betatron emission as a diagnostic for injection and acceleration mechanisms in laser-plasma accelerators

Betatron x-ray emission in laser-plasma accelerators is a promising compact source that may be an alternative to conventional x-ray sources, based on large scale machines. In addition to its potential as a source, precise measurements of betatron emission can reveal crucial information about relativistic laser-plasma interaction. We show that the emission length and the position of the x-ray emission can be obtained by placing an aperture mask close to the source, and by measuring the beam profile of the betatron x-ray radiation far from the aperture mask. The position of the x-ray emission gives information on plasma wave breaking and hence on the laser non-linear propagation. Moreover, the measurement of the longitudinal extension helps one to determine whether the acceleration is limited by pump depletion or dephasing effects. In the case of multiple injections, it is used to retrieve unambiguously the position in the plasma of each injection. This technique is also used to study how, in a capillary discharge, the variations of the delay between the discharge and the laser pulse affect the interaction. The study reveals that, for a delay appropriate for laser guiding, the x-ray emission only occurs in the second half of the capillary: no electrons are injected and accelerated in the first half.Comment: 8 pages, 6 figures. arXiv admin note: text overlap with arXiv:1104.245

On gamma and neutrino radiation from Cyg X-3

The production of high energy gamma and neutrino radiation is studied for Cyg X-3. A heating model is proposed to explain the presence of only one gamma-pulse during 4.8 h period of the source. The acceleration mechanisms are discussed. High energy neutrino flux from Cyg X-3 is calculated