Speaker
Description
Characteristic gamma-ray lines can be measured from nuclear processes and from positron annihilations in cosmic sites. Nuclear transitions may originate from radioactive by-products of nucleosynthesis, or from energetic collisions at MeV to GeV energies. Positrons may originate from pair production in strong fields or from nuclear decays. Both the nuclear decays and the annihilations of positrons create unique signals as they occur, and are largely independent of thermodynamic variables of their environment, thus making them unique messengers of their exotic sources. In this talk we discuss how gamma-ray spectroscopy has succeeded to measure characteristic lines from radio-isotopes $^{56}$Ni, $^{44}$Ti, $^{60}$Fe, $^{26}$Al, all produced from massive stars and their core-collapse supernovae, and how annihilations of positrons have been observed throughout our Galaxy and in transient sources. We discuss which aspects of massive-star structure and evolution and of black-hole formation are accessible to gamma-ray spectroscopy, and the instrumental situation and prospects for such observational efforts.