ar X iv : a st ro - p h / 02 07 47 5 v 1 2 2 Ju l 2 00 2 SPI SCIENCE PROSPECTS

With the launch of ESA's INTEGRAL satellite in october 2002, a gamma-ray observatory will become available to the scientific community that combines imaging and spectroscopic capacities in the 20 keV to 10 MeV energy range. In this paper, we summarise the science prospects of the SPI spectrometer aboard INTEGRAL which provides unprecedented spectral resolution with good imaging capabilities and high sensitivity. Emphasise will be given to the key objectives of SPI which are the determination of the galactic positron origin, the study of galactic star formation and nucleosynthesis activity via the radioactive trace isotopes 26 Al and 60 Fe, and the study of nucleosynthesis in supernova explosions and the gamma-ray line emission from their remnants. 1 The SPI telescope on INTEGRAL Due to continuing progress in instrumentation, the field of gamma-ray line astronomy has become a new complementary window to the universe. With the COMPTEL and OSSE telescopes onboard CGRO, the sky has been imaged for the first time in the light of gamma-ray lines, leading to maps of 511 keV annihilation radiation and 26 Al 1.809 MeV emission. New gamma-ray lines have been discovered, such as the 1.157 MeV line from 44 Ti or several decay lines from 56 Co and 57 Co. Gamma-ray lines probe aspects of nucleosynthesis, stellar evolution, and supernova physics that are difficult to access by other means. Additionally, they provide tracers of galactic activity and improve our understanding of the interstellar recycling processes. With the upcoming INTEGRAL satellite, foreseen for launch in october 2002, ESA provides a gamma-ray observatory to the scientific community that combines imaging and spectroscopic capacities in the 20 keV to 10 MeV energy range. INTEGRAL is equipped with two gamma-ray telescopes, optimised for high-resolution imaging (IBIS) and high-resolution spectroscopy (SPI), supplemented by two X-ray monitors (JEM-X) and an optical monitor (OMC). With respect to precedent instruments, the INTEGRAL telescopes provide enhanced sensitivity together with improved angular and spectral resolution. In particular, SPI will map gamma-ray lines with an angular resolution of about 2 • and a spectral resolution of E/∆E ∼ 500 (at 1 MeV), corresponding to Doppler velocities of ∼ 600 km s −1. Consequently, gamma-ray line astrophysics figures among the prime objectives of this telescope. SPI consists of a pixelised gamma camera with a geometrical area of ∼ 500 cm −2 , made of 19 hexagonal high-purity germanium crystals cooled actively to a temperature of ∼ 85 K. …