Origin of Galactic Cosmic Rays

Abstract

The origin of the bulk of cosmic rays (CRs) observed at Earth is the topic of a century long investigation, paved with successes and failures. From the energetic point of view, supernova remnants (SNRs) remain the most plausible sources of CRs up to rigidity ∼ 10 6 − 10 7 GV. This confidence somehow resulted in the construction of a paradigm, the so-called SNR paradigm: CRs are accelerated through diffusive shock acceleration in SNRs and propagate diffusively in the Galaxy in an energy dependent way. Qualitative confirmation of the SNR acceleration scenario has recently been provided by gamma ray and X-ray observations. Diffusive propagation in the Galaxy is probed observationally through measurement of the secondary to primary nuclei flux ratios (such as B/C). There are however some weak points in the paradigm, which suggest that we are probably missing some physical ingredients in our models. The theory of diffusive shock acceleration at SNR shocks predicts spectra of accelerated particles which are systematically too hard compared with the ones inferred from gamma ray observations. Moreover, hard injection spectra indirectly imply a steep energy dependence of the diffusion coefficient in the Galaxy, which in turn leads to anisotropy larger than the observed one. Moreover recent measurements of the flux of nuclei suggest that the spectra have a break at rigidity ∼ 200 GV, which does not sit well with the common wisdom in acceleration and propagation. In this paper I will review these new developments and suggest some possible implications.

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

“…Cosmic rays (CRs) have been known and studied for more than a century now (see e.g. Amato (2014) and Blasi (2013) for recent reviews). 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 (positrons and anti-protons, 1 ‰).…”

Superdiffusive transport in laboratory and astrophysical plasmas

“…Helder et al. 2012; Blasi 2013; Amato 2014). This association has gained important observational support in recent years thanks to the data collected by X-ray telescopes such as Chandra and XMM-Newton and gamma-ray telescopes such as Fermi and AGILE, with the former instruments showing accelerated electrons with energies up to tens of TeV (Reynolds et al.…”

The role of re-acceleration in the understanding of Cosmic-Ray direct and indirect data

“…The role of pre-existing CRs DSRe was analyzed even separately from the γ-ray astronomy. The huge amount of data collected during the last decade, indeed, has unveiled a lot of tricky features in CR particle spectra [15,8,27]. One of this feature is the hardening of both primary and secondary spectra detected by PAMELA and AMS-02 above a rigidity of 300 GV [16,23].…”

“…Cosmic Ray (CR) particles are mainly protons and heavy nuclei with an energy spectrum extended from some GeV to 10 20 eV. They follow a power-law distribution with several important features: a bending at the lowest energies due to solar modulation, a steepening of the spectrum from E −2.7 to E −3.1 at E ∼ 10 15 eV (knee) and a hardening at energies of about 10 18 eV (ankle) [15,8,27, for a review]. We know that these particles are accelerated through the first order Fermi mechanism (or diffusive shock acceleration, DSA) [24,25,10] but the issue of their origin is still open to discussion.…”