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

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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 10 15 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 10 19 eV . (See PHYSICS TODAY, January 1998, page 31). The power of about 10 41 ergs / s required to maintain the cosmic rays throughout the Galaxy is most likely supplied by supernovae (figure 1). With a Galacticsupernova rate of roughly three per century, the required energy per supernova is about 10 50 ergs , which is about 10% of the kinetic energy of the expanding supernovaejecta. Shock acceleration in the supernova blast wave driven by the ejecta could impart such a proportion of the available kinetic energy to cosmic rays. Recent observations of Li, Be and B abundances in halo stars formed in the early Galaxy shed new light on the source of cosmic rays, suggesting acceleration from the ejecta of supernovae.