Understanding the origin of the positron annihilation line and the physics of supernova explosions

Abstract

Nuclear astrophysics, and particularly nuclear emission line diagnostics from a variety of cosmic sites, has remained one of the least developed fields in experimental astronomy, despite its central role in addressing a number of outstanding questions in modern astrophysics. Radioactive isotopes are co-produced with stable isotopes in the fusion reactions of nucleosynthesis in supernova explosions and other violent events, such as neutron star mergers. The origin of the 511 keV positron annihilation line observed in the direction of the Galactic Center is a 50-year-long mystery. In fact, we still do not understand whether its diffuse large-scale emission is entirely due to a population of discrete sources, which are unresolved with current poor angular resolution instruments at these energies, or whether dark matter annihilation could contribute to it. From the results obtained in the pioneering decades of this experimentally-challenging window, it has become clear that some of the most pressing issues in high-energy astrophysics and astro-particle physics would greatly benefit from significant progress in the observational capabilities in the keV-to-MeV energy band. Current instrumentation is in fact not sensitive enough to detect radioactive and annihilation lines from a wide variety of phenomena in our and nearby galaxies, let alone study the spatial distribution of their emission. In this White Paper (WP), we discuss how unprecedented studies in this field will become possible with a new low-energy gamma-ray space experiment, called ASTENA (Advanced Surveyor of Transient Events and Nuclear Astrophysics), which combines new imaging, spectroscopic and polarization capabilities. In a separate WP (Guidorzi et al. 39), we discuss how the same mission concept will enable new groundbreaking studies of the physics of Gamma–Ray Bursts and other high-energy transient phenomena over the next decades.

Opening the path to hard X-/soft gamma-ray focussing: the ASTENA-pathfinder mission

“…High energy astrophysics has still unanswered questions mainly due to the poor sensitivity of the present instrumentation: transient events like gamma-ray bursts and in particular their X-ray afterglow emission, blazars and magnetars spectra are just a few examples of science that would benefit from higher sensitive instruments. An exhaustive review of the science that can be tackled with future focusing optics can be found in [1,2]. A substantial increase in sensitivity can be achieved by enabling hard x-ray focusing.…”

Telescope Concepts in Gamma-Ray Astronomy

“…An important part of these concepts is the formation flight of two or more spacecrafts in sync. Another possibility to achieve a much enhanced sensitivity would also to use a deployable boom, such as used in the AS-TENA proposal [125,126]. Here, the goal is not to approach the diffraction limit, but to construct a design feasible with current technologies.…”

Recent developments in Laue lens manufacturing and their impact on imaging performance

“…The final objective of our R&D is to develop a Laue lens working in the 50 to 700 keV energy range as that of the narrow field telescope proposed for the ASTENA mission concept. 3,4 Such a Laue lens has an aperture diameter of ∼3.2 m and an overall geometric area of 7 m 2 (Fig. 2).…”