Abstract
Shear flows are ubiquitously present in space and astrophysical plasmas. This paper highlights the central idea of the non-thermal acceleration of charged particles in shearing flows and reviews some of the recent developments. Topics include the acceleration of charged particles by microscopic instabilities in collisionless relativistic shear flows, Fermi-type particle acceleration in macroscopic, gradual and non-gradual shear flows, as well as shear particle acceleration by large-scale velocity turbulence. When put in the context of jetted astrophysical sources such as Active Galactic Nuclei, the results illustrate a variety of means beyond conventional diffusive shock acceleration by which power-law like particle distributions might be generated. This suggests that relativistic shear flows can account for efficient in-situ acceleration of energetic electrons and be of relevance for the production of extreme cosmic rays.
Highlights
Shear flows are naturally expected in a variety of astrophysical environments
As described in this review, a variety of processes may be operative in astrophysical shear flows and facilitate particle transport and energization
Key results include the self-consistent generation of electromagnetic micro-turbulence and supra-thermal particle distributions as well as efficient Fermi-type particle acceleration in relativistic shearing flows
Summary
Shear flows are naturally expected in a variety of astrophysical environments. Prominent examples include the rotating accretion flows around compact objects and the relativistic outflows (jets) in gamma-ray bursts (GRBs) or Active Galactic Nuclei (AGN) [1]. Various relativistic hydrodynamic and magneto-hydrodynamic simulations of two-component (spine-sheath/layer) and rotating AGN-type jets have been carried out to study their stability properties (e.g., [13,14,15,16,17,18,19]), indicating for example that the presence of a sheath has a stabilising effect on the jet (e.g., [13,14,15,16,17,18,19]) All this suggests that a transversal velocity stratification is a generic feature of AGN-type jets. The fast jets of powerful AGN are observed to extend over several hundreds of kilo-parsec (kpc), with bright hot spots being formed and significant backflows induced when these jets eventually terminate in the intergalactic medium Though these jets are associated with large fluid. This review shows that a variety of processes beyond conventional diffusive shock acceleration could contribute to the efficient energization of particles in jetted astrophysical sources
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