Cosmic Rays, Nuclear Gamma Rays and the Origin of the Light Elements

Abstract

The origin of cosmic rays has been a major mystery in astrophysics for nearly a century. However, any lingering doubt about whether the bulk of the cosmic rays (those with energies below about 1015 eV) are Galactic or extragalactic has been removed in the 1990s in favor of a Galactic origin. The question has been settled by gamma-ray observations made by the Energetic Gamma Ray Experiment Telescope on the Compton Gamma Ray Observatory. The EGRET observations showed that the cosmic-ray energy density in a nearby galaxy—the Small Magellanic Cloud—is much lower than that found locally in our own Galaxy and is thus inconsistent with a uniform extragalactic density. This discovery, of course, does not preclude an extragalactic origin for the very highest energy cosmic rays, which are observed above about 1019 eV. (See PHYSICS TODAY, January 1998, page 31). The power of about 1041 ergs/s required to maintain the cosmic rays throughout the Galaxy is most likely supplied by supernovae (figure 1). With a Galactic supernova rate of roughly three per century, the required energy per supernova is about 1050 ergs, which is about 10% of the kinetic energy of the expanding supernova ejecta. Shock acceleration in the supernova blast wave driven by the ejecta could impart such a proportion of the available kinetic energy to cosmic rays.