Abstract
The energy range between about 100 keV and 1 GeV is of interest for a vast class of astrophysical topics. In particular, (1) it is the missing ingredient for understanding extreme processes in the multi-messenger era; (2) it allows localizing cosmic-ray interactions with background material and radiation in the Universe, and spotting the reprocessing of these particles; (3) last but not least, gamma-ray emission lines trace the formation of elements in the Galaxy and beyond. In addition, studying the still largely unexplored MeV domain of astronomy would provide for a rich observatory science, including the study of compact objects, solar- and Earth-science, as well as fundamental physics. The technological development of silicon microstrip detectors makes it possible now to detect MeV photons in space with high efficiency and low background. During the last decade, a concept of detector (“ASTROGAM”) has been proposed to fulfil these goals, based on a silicon hodoscope, a 3D position-sensitive calorimeter, and an anticoincidence detector. In this paper we stress the importance of a medium size (M-class) space mission, dubbed “ASTROMEV”, to fulfil these objectives.
Highlights
Gamma-ray astronomy has experienced a period of impressive scientific advances and successes during the last decade
This fact makes the MeV energy region of paramount importance for the study of radiating, nonthermal particles and for distinguishing leptonic from hadronic processes. This energy domain covers the crucial range of nuclear gamma-ray lines produced by radioactive decay, nuclear collision, positron annihilation, or neutron capture, which makes it as special for high-energy astronomy as optical spectroscopy is for phenomena related to atomic physics
By deciphering many aspects of particle acceleration in the Universe, we address the question of why the energy distribution is so unbalanced: only a few particles carry an extreme share of the available energy, and by their feedback they shape numerous cosmic objects
Summary
Gamma-ray astronomy has experienced a period of impressive scientific advances and successes during the last decade. Many of the most spectacular objects in the Universe have their peak emissivity at photon energies between 0.2 MeV and 100 MeV (e.g. gamma-ray bursts, blazars, pulsars, etc.), so it is in this energy band that essential physical properties of these objects can be studied most directly This energy range is known to feature spectral characteristics associated with gamma-ray emission from pion decay, indicating hadronic acceleration. ASTROMEV will have optimal sensitivity and energy resolution to detect line emissions from 511 keV up to 10 MeV, and a variety of issues will be resolved, in particular: (1) origin of the gamma-ray and positron excesses toward the Galactic inner regions; (2) determination of the astrophysical sources of the local positron population from a very sensitive observation of pulsars and supernova remnants (SNRs). The γ -ray data will provide a much better understanding of Type Ia supernovae and their evolution with look-back time and metallicity, which is a pre-requisite for their use as standard candles for precision cosmology
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