GRI: THE GAMMA-RAY IMAGER MISSION

With the INTEGRAL observatory, ESA has provided a unique tool to the astronomical community revealing hundreds of sources, new classes of objects, extraordinary views of antimatter annihilation in our Galaxy, and fingerprints of recent nucleosynthesis processes. While INTEGRAL provides the global overview over the soft gamma-ray sky, there is a growing need to perform deeper, more focused investigations of gamma-ray sources. In soft X-rays a comparable step was taken going from the Einstein and the EXOSAT satellites to the Chandra and XMM/Newton observatories. Technological advances in the past years in the domain of gamma-ray focusing using Laue diffraction have paved the way towards a new gamma-ray mission, providing major improvements regarding sensitivity and angular resolution. Such a future GammaRay Imager will allow the study of particle acceleration processes and explosion physics in unprecedented detail, providing essential clues on the innermost nature of the most violent and most energetic processes in the Universe. 1 From INTEGRAL to GRI Following 4 years of successful operations, INTEGRAL has significantly changed our vision of the gamma-ray sky. The telescopes aboard the satellite have revealed hundreds of sources of different types, new classes of objects, extraordinary and puzzling views of antimatter annihilation in our Galaxy, and fingerprints or recent nucleosynthesis processes. With the wide fields of view of the IBIS and SPI telescopes, INTEGRAL is an exploratory-type mission that allows extensive surveys of the hard X-ray and soft gamma-ray sky, providing a census of the source populations and first-ever allsky maps in this interesting energy range. The good health of the instruments allows to continue the exploration during the upcoming years, enabling INTEGRAL to provide the most complete and detailed survey ever, which will be a landmark for the discipline throughout the next decades. Based on the INTEGRAL discoveries and achievements, there is now a growing need to perform more focused studies of the observed phenomena. High-sensitivity investigations of point sources, such as compact objects, pulsars, and active galactic nuclei, should help to uncover their yet poorly understood emission mechanisms. A deep survey of the galactic bulge region with sufficiently high-angular resolution should shed light on the still mysterious source of positrons. And a sensitivity leap in the domain of gamma-ray lines should allow the detection of nucleosynthesis products in individual supernova events, providing direct insights into the physics of the exploding stars. Technological advances in the past years in the domain of gamma-ray focusing using Laue diffraction have paved the way towards a new gamma-ray mission that can fulfil these requirements. Laboratory work and balloon campaigns have provided the proof-of-principle for using Laue lenses as focusing devices in gammaray telescopes (von Ballmoos et al. 2004; Halloin et al. 2004), and concept studies by CNES and ESA have demonstrated that such an instrument is technically feasible and affordable (Duchon et al. 2006; Brown 2005). Complemented by a hard X-ray telescope, either based on a coded mask or a multilayer mirror, a broad-band energy coverage can be achieved that allows detailed studies of astrophysical sources at unprecedented sensitivity and angular resolution, from a few tens of keV up to at least 1 MeV. 1 Centre d’Etude Spatiale des Rayonnements, 31000 Toulouse, France 2 the GRI consortium is composed of members from the countries Belgium, China, Denmark, France, Germany, Italy, Ireland, Poland, Portugal, Russia, Spain, The Netherlands, United Kingdom, and the United States. A complete list of GRI consortium members can be found on http://gri.rm.iasf.cnr.it/. c © Société Francaise d’Astronomie et d’Astrophysique (SF2A) 2006