Session H16: Future Science and Prospects in MeV Gamma-ray Astrophysics

All-Sky Medium Energy Gamma-ray Observatory (AMEGO) - A discovery mission for the MeV gamma-ray band

The MeV domain is one of the most underexplored windows on the Universe. From astrophysical jets and extreme physics of compact objects to a large population of unidentified objects, fundamental astrophysics questions can be addressed by a mission that opens a window into the MeV range. AMEGO is a wide-field gamma-ray telescope with sensitivity from ~200 keV to >10 GeV. AMEGO provides three new capabilities in MeV astrophysics: sensitive continuum spectral studies, polarization measurements, and nuclear line spectroscopy. AMEGO will consist of four hardware subsystems: a double-sided silicon strip tracker with analog readout, a segmented CZT calorimeter, a segmented CsI calorimeter and a plastic scintillator anticoincidence detector, and will operate primarily in an all-sky survey mode. In this presentation we will describe the AMEGO mission concept and scientific performance.   

Evaluation of the Scientific Capabilities of the AMEGO instrument

The gamma-ray energy range from several hundred keV to a hundred MeV has remained largely unexplored since the observations by instruments on the Compton Gamma-Ray Observatory (1991- 2000) and on INTEGRAL (since 2002). This energy range is particularly challenging because it is firmly in the Compton-dominated regime where the interaction cross section is minimized. Accurate measurements are critical for answering a broad range of astrophysical questions. To address these questions, we are developing AMEGO: All-sky Medium Energy Gamma-ray Observatory, to investigate the energy range from 200 keV to \textgreater 10 GeV with good energy and angular resolution and with sensitivity approaching a factor of 20-50 better than previous measurements. This instrument will be capable of measuring both Compton-scattering events at lower energies and pair-production events at higher energies.~To achieve these ambitions goals Monte Carlo (MC) simulations will play a crucial role guiding the design of AMEGO. I will present an overview of the AMEGO scientific simulation campaign using the Medium-Energy Gamma-ray Astronomy library (MEGAlib) framework, as well as the initial results for~effective area and angular resolution, as well as sensitivity projections.   

MeV Astrophysics in the Multimessenger Era

AMEGO is a proposed probe-class mission that will observe the MeV band in unprecedented detail, and will be crucial in the multimessenger era. With the Laser Interferometer Gravitational-Wave Observatory's detection of gravitational waves in 2015 all four astrophysical messengers have now been directly observed. There have been two astrophysical events detected with two messengers: SN1987A in neutrinos and photons, and the merging of two neutron stars as GW170817, GRB 170817A, and the resulting kilonova. We will discuss the multimessenger science that is possible with ground-based gravitational wave detectors and MeV astronomy, as well as broader multimessenger prospects.   

Shining Light on Dark Matter in the MeV Regime

For several decades, the WIMP miracle has served as a driving inspiration for studying dark matter models at mass scales of ~100 GeV. Recently, however, a combination of indirect, direct, and collider searches have placed strong constraints on GeV dark matter particles. Interestingly, the vast majority of these methods become relatively insensitive to dark matter particles with energies below ~1 GeV, while several of the same theoretical motivations for studying GeV-scale dark matter still motivate similar searches in the MeV regime. In this talk, I will discuss the prospects for indirectly detecting MeV dark matter, focusing first on the motivations and challenges for dark matter model building at this scale, before discussing in detail the unique indirect detection signatures present in this regime. I will conclude by presenting a plan of attack aimed and efficiently transferring our understanding of GeV indirect detection techniques into the MeV range.   

Understanding Element Formation in Dynamic Environments with MeV Gamma-ray Spectroscopy

The MeV gamma-ray band is the only region of the electromagnetic spectrum where we can directly measure spectral lines from nuclear excitations and de-excitations. Gamma-ray lines have been detected from radioactive isotopes produced in nuclear burning inside stars and supernovae, and from energetic-particle interactions in solar flares. Isotopes with relatively short half lives pinpoint to location of production - e.g. 56Ni directly reflects the source of supernova light. Conversely, Isotopes with long half lives such as 26Al and 60Fe are dispersed in interstellar space from massive-star nucleosynthesis over millions of years, and provide an integrated measure of stellar activity in our Galaxy. Combined, observations of MeV lines can be used to both understand and trace element formation in individual sources and to trace the past history of these explosive events.   

Probing Cosmic-ray Physics in Starburst Galaxies With Future High-Energy Missions

High-energy observations of non-thermal diffuse emission star-forming galaxies provide an excellent opportunity to study cosmic-ray physics in a variety of interstellar environments. Recent advances in gamma- ray astronomy in the GeV and TeV band have resulted in the first detections of non-AGN galaxies outside of the Milky Way, as well as providing more detailed measurements of the non-thermal diffuse emission within the Galaxy. However, the physical mechanism(s) involved in producing the broadband non-thermal diffuse spectrum is difficult to determine solely from gamma-ray observations, particularly for star-forming galaxies other than our own. Recent NuSTAR observations of starburst galaxies such as NGC 253 and M82 enable the most sensitive search to date for their diffuse inverse Compton emission in the hard X- ray band (10-30 keV), which in turn will constrain the role of hadronic and leptonic interactions in producing the GeV emission. We present the latest results from detailed broadband spectral modeling of NGC 253 from keV to TeV energies and discuss implications for future hard X-ray and MeV missions such as HEX-P and AMEGO.   

Development of the AMEGO Subsystems

The All-sky Medium Energy Gamma-ray Observatory (AMEGO) is a probe-class mission in consideration for the 2020 decadal review designed to operate at energies from $\sim$ 200~keV to > 10 GeV. Both Compton scattering and pair-production events must be considered in the AMEGO design since the interaction cross section has a crossover at $\sim$ a few MeV. AMEGO is made of four major subsystems: a plastic anticoincidence detector for rejecting cosmic-ray events, a silicon tracker for measuring the energies of Compton scattered electrons and pair-production products, a CZT calorimeter for measuring the energy and location of Compton scattered photons, and a CsI calorimeter for measuring the energy of the pair-production products at high energies. The prototype subsystems are under development at the NASA Goddard Space Flight Center and the Naval Research Lab; in this contribution we provide details on the development of the different subsystems in preparation for beam tests and a balloon flight.