Cosmic Rays: Recent Progress and some Current Questions

TL;DR

Recent advances in cosmic ray research have improved understanding of the spectrum's features, origins, and acceleration mechanisms, with new evidence supporting supernova remnants as primary sources and ongoing debates about the transition to extragalactic cosmic rays.

Contribution

This paper synthesizes recent observational and theoretical developments, highlighting progress in understanding cosmic ray origins, spectrum features, and the transition to extragalactic sources.

Findings

KASCADE observations support a sharp rigidity cut-off at the knee

Supernova remnants likely accelerate particles to similar rigidities

No conflict observed with GZK cutoff at highest energies

Abstract

Recent progress suggests we are moving towards a quantitative understanding of the whole cosmic ray spectrum, and that many bumps due to different components and processes hide beneath a relatively smooth total flux between knee and ankle. The knee is much better understood: the KASCADE observations support a rather sharp rigidity cut-off; while theoretical developments (strong magnetic field generation) indicate that supernova remnants (SNR) of different types should indeed accelerate to a very similar rigidity. X-ray and TeV observations of shell-type SNR produce evidence in favour of acceleration at their outer boundaries. There is some still-disputed evidence that the transition to extragalactic cosmic rays has already occurred just above 10**17 eV, unmarked by an "ankle", in which case the whole spectrum can be well described by adding a single power-law source spectrum from many extragalactic sources (but modified by energy losses) to the SNR pre-knee component, if the sources photodisintegrate all nuclei. At the highest energy, the experiments using fluorscence light to calibrate energy do not yet show any conflict with an expected GZK "termination". GRBs seem not to make a significant contribution.