The origin of galactic cosmic rays

TL;DR

This paper reviews the history and current understanding of galactic cosmic rays, highlighting the progress and challenges in confirming their acceleration mechanisms and sources, mainly supernova remnants.

Contribution

It provides a comprehensive review of recent progress and identifies future directions to obtain definitive evidence for the origin of galactic cosmic rays.

Findings

Progress in theory and observations but no conclusive proof

Circumstantial evidence linking SNRs to CR acceleration

Need for new observational strategies to find definitive proof

Abstract

Initial discovery of CRs dates back to a century ago (1912). Their identification as particles rather than radiation dates to about 20 years later and in 20 more years also the first suggestion that they were associated with SNRs was in place. The basic mechanism behind their acceleration was suggested almost 40 years ago. Much work has been done since then to the aim of proving that both the acceleration mechanism and site are well understood, but no definite proof has been obtained: in spite of impressive progress of both theory and observations, the evidence in support of the commonly accepted interpretation is only circumstantial. In the following I will try to make the point on where we stand in terms of how our theories confront with data: I will review recent progress on the subject and try to point to avenues to pursue in order to gather new proofs, if not smoking gun evidence of the origin of Galactic Cosmic Rays.