Integral and Nuclear Astrophysics

We briefly review the fundamentals of nuclear gamma-ray line astronomy (radioactive astronomy), focusing on its role to decipher the intimate physics of supernovae, either immediatly (via 56 Co) or after a time delay (via 44 T i). All kinds of super-novae can be in principle tested through their radioactivities and their associated gamma-ray lines. Dedicated to the spectroscopy and imaging of celestial sources in the 15 keV to 10 MeV band, the ESA scientific observatory INTEGRAL will open a golden age of nuclear astrophysics in Europe. 1 Why a new gamma-ray astronomy mission? Gamma-ray astronomy explores the most energetic phenomena that occur in nature and addresses some of the most fundamental problems in astrophysics. It embraces a great variety of gamma-ray continuum and gamma-ray line production processes: nuclear excitation, radioactivity, positron annihilation and Compton scattering; and an even a greater diversity of astrophysical objects and phenomena: nucleosynthesis, nova and supernova explosions, the interstellar medium, cosmic-ray interactions and sources, neutron stars, black holes, gamma-ray bursts, active galactic nuclei and the cosmic gamma-ray background. Not only do gamma rays allow us to see deeper into these objects, but the bulk of the power radiated by them is often at gamma-ray energies. In the low-energy gamma-ray band, line-forming processes such as nuclear excitation, radioactivity, positron annihilation, cyclotron emission and absorption become important, and when used as astrophysical tools. Unique astrophysical information is contained in the spectral shift, line width, and line profiles. Detailed studies of these processes require the resolving power of a germanium spectrometer. Lower-resolution spectrometers (e.g. SIGMA, OSSE, COMPTEL) did not have sufficient energy resolution to permit a study of the parameters of these lines. The last high-resolution space instrument, that on HEAO-3 in 1979-80, was 100 times less sensitive than required to tackle the scientific subjects outlined below. Solid observational and theoretical grounds already exist for predicting detectable line emission from such