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.
Highlights
Cosmic rays (CRs) have been known and studied for more than a century (see e.g. Amato (2014) and Blasi (2013) for recent reviews)
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
They are highly energetic charged particles, mainly protons and He nuclei, with a minor fraction of heavier nuclei (1 %), electrons (2 %) and anti-matter particles. Their origin was associated with supernova (SN) explosions in the Galaxy already in the 1930s and the mechanism by which they would be accelerated up to very high energies (VHEs) in the blast waves emerging from SN explosions was proposed in the late 1970s
Summary
Cosmic rays (CRs) have been known and studied for more than a century (see e.g. Amato (2014) and Blasi (2013) for recent reviews). The most important in terms of implications on our understanding of CR acceleration and transport are: (1) the detection of a hardening in the spectra of protons, He nuclei and virtually all primary nuclei (Ahn et al 2010; Adriani et al 2011; Aguilar et al 2015a,b), at R ≈ 300 GV, where R = cp/Ze is the particle rigidity, with p the particle momentum, Z its atomic number, e the electron charge and c the speed of light; (2) the hardening found in the spectrum of secondary CRs at R ≈ 200 GV, with a change of spectral slope that is approximately twice as large as that of primaries (Aguilar et al 2018); (3) the energy dependence of the ratio between secondary and primary CRs, that has been measured with excellent statistics up to TV rigidities (Aguilar et al 2016b) and provides invaluable constraints on the energy dependence of particle transport in the Galaxy; (4) a rise in the fraction of positrons-to-electrons at energies larger than 30 GeV (Adriani et al 2009; Aguilar et al 2013), possibly suggesting the presence of an additional source of positrons in the Galaxy; (5) the spectrum of anti-p which is unexpectedly very close to that of protons and positrons (Aguilar et al 2016a)
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