Gamma-ray lines from cosmic radioactivity

Abstract

Gamma-ray spectroscopy holds great promise for probing nucleosynthesis in individual stellar explosions via short-lived radioactivity, and for measuring current global Galactic nucleosynthesis with longer-lived radioactivity. The former case was realized first for a Type II supernova when both 56Co and 57Co were detected in SN 1987A. These provide unprecedented constraints on models of core-collapse explosions. In the second category, 26Al in the Galaxy is now well observed and comes from massive stars, either before or during core collapse explosions, or from relatively massive AGB stars. We currently try to use these observations to constrain massive star evolution, supernova nucleosynthesis, and the Galactic supernova rate. Type Ia supernovae, thought to be entirely thermonuclear events, have not yet been definitively detected in γ-rays (although there is a hint of a signal from SN 1991T.) Ultimately, γ-ray measurements will confirm or disprove their thermonuclear character, probe the nuclear burning conditions, and help evaluate their contributions to Galactic nucleosynthesis. Positron annihilation radiation is also well-observed from the Galactic plane and bulge. All types of supernovae are plausibly significant contributors via their ejected positron emitters. By sorting out the contributions of the many possible sources, we can use these observations to constrain supernova nucleosynthesis, and we might obtain important clues to the progenitors of thermonuclear supernovae. All supernova types might contribute to a diffuse glow of 60Fe γ-ray lines. Current limits are very close to expected emission levels. Remnants of any type of age less that a few centuries might be detectable as individual spots of 44Ti γ-ray line emission. It is quite surprising that the brightest such spot is apparently the Cas A remnant, a relatively old one. That younger ones are not detected provides constraints on the combination of nucleosynthesis yields and supernova rates. Here we review the status of observations of γ-ray lines from these sources, discuss their implications, and briefly outline prospects for the future.