Supernova Puzzles and Gamma - Ray Astronomy

The signiicance of supernovae for the Compton GRO has lain primarily in the large amount of radioactivity that they eject. This activity and its daughters nd their way into the interstellar medium and into other arenas of astrophysics. Direct measures of the quantity of such activity carries profound information that is not available from observations of stable nuclei. CGRO has had both success and disappointment: success at detecting or placing important limits to radiation from three of the brightest extragalactic supernovae of recent decades, disappointment that those three lay at the \plate limit" for OSSE and COMPTEL, and that nature has not smiled even wider upon us with a Galactic Type II or a Type Ia within 10 Mpc. While we review this situation we call attention to some exciting puzzles about supernovae and radioactive production within them. These puzzles place the CGRO mission into the larger framework of the science of cosmic radioactivity. The universe should be profoundly radioactive. Clayton & Silk (1969) pointed out that 56 Fe is overwhelmingly the most proliic activator because of its high abundance and because the parenting 56 Co decay occurs in a relatively transparent state of the expansion, owing to its long half-life (77d). The et al. (1993) reevaluated the relevant emission quantities in their assessment of this-ray background. They nd that 60% of the total 847 keV-rays from Type Ia 56 Co escape, for example, and argue that 2/3 of 56 Co production in our own Galaxy has resulted from Type Ia