13 research outputs found
Fermi acceleration in astrophysical jets
We consider the acceleration of energetic particles by Fermi processes (i.e.,
diffusive shock acceleration, second order Fermi acceleration, and gradual
shear acceleration) in relativistic astrophysical jets, with particular
attention given to recent progress in the field of viscous shear acceleration.
We analyze the associated acceleration timescales and the resulting particle
distributions, and discuss the relevance of these processes for the
acceleration of charged particles in the jets of AGNs, GRBs and microquasars,
showing that multi-component powerlaw-type particle distributions are likely to
occur.Comment: 6 pages, one figure; based on talk at "The multimessenger approach to
unidentified gamma-ray sources", Barcelona/Spain, July 2006; accepted for
publication in Astrophysics and Space Scienc
Topical Issues for Particle Acceleration Mechanisms in Astrophysical Shocks
Particle acceleration at plasma shocks appears to be ubiquitous in the
universe, spanning systems in the heliosphere, supernova remnants, and
relativistic jets in distant active galaxies and gamma-ray bursts. This review
addresses some of the key issues for shock acceleration theory that require
resolution in order to propel our understanding of particle energization in
astrophysical environments. These include magnetic field amplification in shock
ramps, the non-linear hydrodynamic interplay between thermal ions and their
extremely energetic counterparts possessing ultrarelativistic energies, and the
ability to inject and accelerate electrons in both non-relativistic and
relativistic shocks. Recent observational developments that impact these issues
are summarized. While these topics are currently being probed by
astrophysicists using numerical simulations, they are also ripe for
investigation in laboratory experiments, which potentially can provide valuable
insights into the physics of cosmic shocks.Comment: 13 pages, no figures. Invited review, accepted for publication in
Astrophysics and Space Science, as part of the HEDLA 2006 conference
proceeding
Pulsar Wind Nebulae with Bow Shocks: Non-thermal Radiation and Cosmic Ray Leptons
Pulsars with high spin-down power produce relativistic winds radiating a non-negligible fraction of this power over the whole electromagnetic range from radio to gamma-rays in the pulsar wind nebulae (PWNe). The rest of the power is dissipated in the interactions of the PWNe with the ambient interstellar medium (ISM). Some of the PWNe are moving relative to the ambient ISM with supersonic speeds producing bow shocks. In this case, the ultrarelativistic particles accelerated at the termination surface of the pulsar wind may undergo reacceleration in the converging flow system formed by the plasma outflowing from the wind termination shock and the plasma inflowing from the bow shock. The presence of magnetic perturbations in the flow, produced by instabilities induced by the accelerated particles themselves, is essential for the process to work. A generic outcome of this type of reacceleration is the creation of particle distributions with very hard spectra, such as are indeed required to explain the observed spectra of synchrotron radiation with photon indices Γ≲ 1.5. The presence of this hard spectral component is specific to PWNe with bow shocks (BSPWNe). The accelerated particles, mainly electrons and positrons, may end up containing a substantial fraction of the shock ram pressure. In addition, for typical ISM and pulsar parameters, the e+ released by these systems in the Galaxy are numerous enough to contribute a substantial fraction of the positrons detected as cosmic ray (CR) particles above few tens of GeV and up to several hundred GeV. The escape of ultrarelativistic particles from a BSPWN—and hence, its appearance in the far-UV and X-ray bands—is determined by the relative directions of the interstellar magnetic field, the velocity of the astrosphere and the pulsar rotation axis. In this respect we review the observed appearance and multiwavelength spectra of three different types of BSPWNe: PSR J0437-4715, the Guitar and Lighthouse nebulae, and Vela-like objects. We argue that high resolution imaging of such objects provides unique information both on pulsar winds and on the ISM. We discuss the interpretation of imaging observations in the context of the model outlined above and estimate the BSPWN contribution to the positron flux observed at the Earth
Turbulence, Magnetic Reconnection in Turbulent Fluids and Energetic Particle Acceleration
Turbulence, Magnetic Reconnection in Turbulent Fluids and Energetic Particle Acceleration
Observational Signatures of Particle Acceleration in Supernova Remnants
We evaluate the current status of supernova remnants as the sources of
Galactic cosmic rays. We summarize observations of supernova remnants, covering
the whole electromagnetic spectrum and describe what these obser- vations tell
us about the acceleration processes by high Mach number shock fronts. We
discuss the shock modification by cosmic rays, the shape and maximum energy of
the cosmic-ray spectrum and the total energy budget of cosmic rays in and
surrounding supernova remnants. Additionally, we discuss problems with
supernova remnants as main sources of Galactic cosmic rays, as well as
alternative sources.Comment: Accepted for publication by Space Science Reviews, 81 page