The Status of Observations and Speculations Concerning Ultra High-Energy Cosmic Rays

Abstract

For the purposes of this chapter I have defined Ultra High-Energy Cosmic Rays (UHECRs) as those cosmic particles or photons that have energies above 3 × 10 eV. This limit is chosen as there has been much speculation (and even much belief) that at higher energies the majority of cosmic rays have their origins outside our galaxy. Although there is a lack of firm observational evidence to support such a hypothesis, it is perfectly possible that either of the exotic regions highlighted in the title of this book, ‘Star Forming Regions and Active Galaxies’, could be the sources. Below, after a brief historical review, I will describe the present status of observations of particles above 3× 10 eV and discuss some of the ideas that have been advanced to explain the observations. There is great uncertainty about the properties of the highest energy cosmic rays. The major questions concerning possible anisotropies in the distribution of arrival directions, the mass composition and the long-sought steepening in the energy spectrum near 10eV (the Greisen-Zatsepin-Kuz’min effect) remain unanswered: but there has been significant progress and there is promise of a rapid increase in our knowledge over the next 10 years. In this article I will outline how UHECRs are detected and review the observations of the energy spectrum, the mass composition and distribution of arrival directions, all three of which needed to be interpreted in a consistent manner. I will also discuss some of the ideas that have been advanced to explain the origin of the highest energy cosmic rays.