The DUAL concept
Gamma-ray astronomy presents an extraordinary scientific potential for the study of the most powerful sources and the most violent events in the Universe. In order to take full advantage of this potential, the next generation of instrumentation for this domain will have to achieve an improvement in sensitivity over present technologies of at least an order of magnitude. The requirements on a future mission in gamma-ray astronomy can be divided into two subsets: a requirement for large-scale exposures, and very deep pointed observations. This duality is naturally addressed by the DUAL mission concept : While an All-Sky Compton Imager (ASCI) accumulates data from the full gamma-ray sky over the entire mission lifetime, two optical modules, the Laue-Lens Optic (LLO) and the Coded-Mask Optic (CMO) focus on selected targets.
During a 3-year exposure of every γ-ray source in the sky, DUAL's All-Sky Compton Imager (ASCI) performs sensitive γ-ray spectroscopy and polarimetry in the energy band 100 keV-10 MeV. The ASCI measures polarization in a large number of γ-ray bursts; it unveils the distribution and ultimately the origin of galactic positrons; it performs a detailed all-sky survey of the radioactive Milky Way, clarifying long-lived activities from supernovae and novae; and it studies excitation lines from as yet unknown MeV cosmic-rays interacting with the interstellar medium. The deep all-sky survey naturally monitors galactic black holes, neutron stars, pulsars, magnetars, and simultaneously, a full sky of Active Galactic Nuclei. DUAL's capability of measuring polarization provides a powerful new diagnostic of magnetic fields - for studying acceleration in neutron star magnetospheres and for their role in the origin of γ-ray emission and jets from accreting BHs, both galactic and extragalactic.
Simultaneous with its all-sky survey, the ASCI serves as a low-background focal plane detector for two optical modules, the Laue-Lens Optic (LLO) and the Coded-Mask Optic (CMO). DUAL will, for the first time ever, make use of focusing optics to concentrate high-energy photons onto a small focal spot. With its Laue-Lens Optic (LLO), during dedicated pointings, DUAL will observe radioactive 56Ni and 56Co in a sizable sample of SNIA at distances of up to 30-40 Mpc. Focusing will bring the long awaited sensitivity leap needed to answer pressing astrophysical questions of our age: What are Type Ia Supernova progenitors? How do their nuclear flames burn in degenerate matter? Are the ancient ones from which we infer an accelerating Universe well calibrated using a local sample?
The Coded-Mask Optic (CMO) dramatically enhances ASCI's imaging capabilities in the Galactic Center and Bulge. During deep, dedicated, observations high angular resolution (10'-40') will be achieved in this single region. DUAL's CMO will be able to disentangle the emission of compact objects in the Galactic Center, and most importantly, map out the, as yet, unresolved bulge of e+-e- annihilation radiation in order to identify the sources of positrons that have puzzled high-energy astronomers for over 30 years.
