On particle acceleration and transport in plasmas in the Galaxy: theory and observations

Abstract

Accelerated particles are ubiquitous in the Cosmos and play a fundamental role in many processes governing the evolution of the Universe at all scales, from the sub-AU scale relevant for the formation and evolution of stars and planets to the Mpc scale involved in Galaxy assembly. We reveal the presence of energetic particles in many classes of astrophysical sources thanks to their production of non-thermal radiation, and we detect them directly at the Earth as cosmic rays. In the last two decades both direct and indirect observations have provided us a wealth of new, high-quality data about cosmic rays and their interactions both in sources and during propagation, in the Galaxy and in the Solar System. Some of the new data have confirmed existing theories about particle acceleration and propagation and their interplay with the environment in which they occur. Some others have brought about interesting surprises, whose interpretation is not straightforward within the standard framework and may require a change of paradigm in terms of our ideas about the origin of cosmic rays of different species or in different energy ranges. In this article, we focus on cosmic rays of galactic origin, namely with energies below a few petaelectronvolts, where a steepening is observed in the spectrum of energetic particles detected at the Earth. We review the recent observational findings and the current status of the theory about the origin and propagation of galactic cosmic rays.

Diffusion of cosmic rays in MHD turbulence with magnetic mirrors

“…This has been a subject of intensive research (Jokipii 1966;Kulsrud & Pearce 1969;Schlickeiser & Miller 1998;Giacalone & Jokipii 1999). The development of modern theories of magnetohydrodynamic (MHD) turbulence over the past few decades (Goldreich & Sridhar 1995;Lazarian & Vishniac 1999;Cho & Vishniac 2000;Maron & Goldreich 2001;Lithwick & Goldreich 2001;Cho et al 2002;Cho & Lazarian 2003;Beresnyak 2014;Kowal et al 2017, see also the book by Beresnyak & Lazarian 2019) leads to a paradigm shift to the standard diffusion models of CRs, whose predictions have been challenged by recent multifrequency observations and direct CR measurements (Nava & Gabici 2013;Orlando 2018;Gabici et al 2019;Amato & Casanova 2021).…”

The Crab Pulsar and Nebula as seen in gamma-rays

“…1 we show the most recent measurements of Crab nebula gamma-ray spectrum, including LHAASO data points, showing emission beyond 1 PeV, about the highest energy we think achievable by galactic accelerators, based on measurements of the cosmic ray spectrum at the Earth (see e.g. [61] for a recent review). Before discussing the most impressive surprises that came from gamma-rays and how they impact our understanding of the Crab nebula, we briefly review the physical picture of the nebular dynamics and emission properties that has been built through time, thanks to constant improvement of the quality of observations, of theories and numerical modeling.…”

The Crab Pulsar and Nebula as Seen in Gamma-Rays

“…In the last 15 years, the advent of the current generation of HE (Fermi-LAT and AGILE) and VHE (MAGIC, VERITAS, H.E.S.S., HAWC, Tibet As-γ, LHAASO) gammaray telescopes has allowed us to gain much deeper insight in the properties of the Crab nebula at these highest energies, and has also brought two big surprises: variability in the MeV range [49][50][51] and detection up to unexpectedly high energies [52]. In Figure 1, we show the most recent measurements of the Crab nebula gamma-ray spectrum, including LHAASO data points, showing emission beyond 1 PeV-about the highest energy we think achievable by galactic accelerators, based on measurements of the cosmic ray spectrum at the Earth (see e.g., [53] for a recent review). Before discussing the most impressive surprises that came from gamma-rays and how they have impacted our understanding of the Crab nebula, we briefly review the physical picture of the nebular dynamics and emission properties that has been built through time, thanks to constant improvements in the quality of observations, theories, and numerical modeling.…”