Multi-scale simulations of particle acceleration in astrophysical systems

Abstract

This review aims at providing an up-to-date status and a general introduction to the subject of the numerical study of energetic particle acceleration and transport in turbulent astrophysical flows. The subject is also complemented by a short overview of recent progresses obtained in the domain of laser plasma experiments. We review the main physical processes at the heart of the production of a non-thermal distribution in both Newtonian and relativistic astrophysical flows, namely the first and second order Fermi acceleration processes. We also discuss shock drift and surfing acceleration, two processes important in the context of particle injection in shock acceleration. We analyze with some details the particle-in-cell (PIC) approach used to describe particle kinetics. We review the main results obtained with PIC simulations in the recent years concerning particle acceleration at shocks and in reconnection events. The review discusses the solution of Fokker–Planck problems with application to the study of particle acceleration at shocks but also in hot coronal plasmas surrounding compact objects. We continue by considering large scale physics. We describe recent developments in magnetohydrodynamic (MHD) simulations. We give a special emphasis on the way energetic particle dynamics can be coupled to MHD solutions either using a multi-fluid calculation or directly coupling kinetic and fluid calculations. This aspect is mandatory to investigate the acceleration of particles in the deep relativistic regimes to explain the highest cosmic ray energies.

Physics and Phenomenology of Weakly Magnetized, Relativistic Astrophysical Shock Waves

“…Particles can gain energy at a relativistic shock front through repeated bounces on the magnetized plasmas up-and downstream of the shock, much as in the well-known subrelativistic first-order Fermi process [11,22,23]. The particle acceleration mechanisms have been reviewed extensively elsewhere [13,14,[24][25][26][27][28][29], so we will simply stress some important features of the relativistic regime: 1. Given that β p ∼ β sh and β sh 1, with β p a particle velocity, accelerated particles do not diffuse spatially in the upstream plasma before returning to the shock front.…”

Simulations of cosmic ray propagation

“…Early theoretical models (Krymskii 1977;Axford et al 1977;Bell 1978;Blandford and Ostriker 1978) as well as recent numerical simulations (e.g. Caprioli and Spitkovsky 2014) confirm this paradigm, see also Marcowith et al (2020) for a recent review. In the Milky Way and most of the star forming galaxies supernova remnants (SNR) are by far the most abundant source of strong shocks that provide the conditions to accelerate CRs up to an energy of approximately 10 15 eV, so the particles up to that energy are likely to be of Galactic origin (Ackermann et al 2013).…”

PIC methods in astrophysics: simulations of relativistic jets and kinetic physics in astrophysical systems

“…Magnetic reconnection is ubiquitous in solar and magnetosphere plasmas (e.g., Lazarian et al 2020;Marcowith et al 2020). It is proposed that it provides an important additional particle acceleration mechanism for AGN and GRB jets (e.g., Giannios et al 2009;Komissarov et al 2009;Giannios 2010;Zhang and Yan 2010;Uzdensky 2011;Granot et al 2011;Granot 2012;McKinney and Uzdensky 2012;Komissarov 2012;Sironi and Giannios 2014;Sironi et al , 2016; Bottom row shows statistics of the acceleration events as a function of simulation time and particle energy.…”

“…The spectrum of accelerated particles achieved with PIC simulations is very informative to understand of shocks and particle acceleration, but further investigations will follow. Marcowith et al (2020) have reviewed a general introduction to the subject of the numerical study of energetic particle acceleration and transport in turbulent astrophysical flows providing an up-to-date status. The subject is also complemented by a short overview of recent progresses obtained in the domain of laserplasma experiments.…”