Cosmic Gamma-Ray Spectroscopy

Penetrating gamma-rays require complex instrumentation for astronomical spectroscopy measurements of gamma-rays from cosmic sources. A combination of multiple-interaction detectors in space and post-processing of detector events on ground have lead to a spectroscopy performance which is now capable to provide new astrophysical insights. Spectral signatures in the MeV regime originate from transitions in atomic nuclei, stimulated by either radioactive decays or high-energy nuclear collisions such as with cosmic rays. Lines have been detected from radioactive isotopes produced in stellar and supernova nuclear burning, and from energetic-particle interactions in solar flares. Radioactive-decay gamma-rays from 56Ni directly reflect the power source of supernova light. 44Ti is produced in core-collapse supernova interiors and the largely unknown and dynamical conditions herein. From 26Al and 60Fe which are distributed in interstellar space from massive-star nucleosynthesis over millions of years. Additionally, nuclear de-excitation lines have been measured in solar-flare events, and convey information about energetic particle production in these outbursts, and their interaction in the solar atmosphere. Annihilating positrons add another very special astrophysical source, which has been puzzling so far, with its characteristic gamma-rays at 511 keV; it has been measured both in such solar flares, and throughout the interstellar medium of our Milky Way galaxy. We discuss instrumentation and data processing for cosmic gamma-ray spectroscopy, and the astrophysical issues and insights from these measurements.