Gamma-Ray Line Observations with the COMPTEL Imaging Telescope

Abstract

The recent nucleosynthesis activity in the Galaxy can be traced through the 1.809 MeV gamma-ray line from radioactive 26Al with its decay time of 1.04 million years. The COMPTEL imaging telescope aboard the Compton Gamma-Ray Observatory performed a survey of the Galactic plane. The resulting map of 1.809 MeV gamma-ray line emission along the Galactic plane shows highly structured emission over a wide longitude range and a marked asymmetry relative to the Galactic Centre. This suggests that the 1.809 MeV emission originates in rather localised regions, not necessarily concentrated in the inner disk of the Galaxy. Smooth intensity distributions as expected for a nova origin are very hard to reconcile with the data. An intermittent nature of both massive-star formation and 26Al line emission, each on a time scale of a few million years, probably determines to a large extent the structured appearance of the 1.809 MeV sky. Nearby sources of 26Al, presumably core-collapse supernovae or Wolf-Rayet stars, may therefore contribute significanuy to the observed intensity distribution. The Vela region in particular shows evidence for a single near-by 26Al source, the Vela supernova remnant. The recently revised and improved distance estimates of <300pc to this supernova remnant result in an observed 26Al mass consistent with core collapse supernova nucleosynthesis models. It appears that the Galaxy-wide total mass of 26Al is not above 1 M⊙, which reduces the yield requirements from candidate source events for the Galaxy-wide component. Therefore a 26Al origin from massive stars is likely. Remaining issues are the Galactic distribution of these objects, and mechanisms for possible clustering of these sources.