SCIENCE CASE

Using Silicon detector technology that is backed by substantial space heritage and industry know-how, e-ASTROGAM will open the MeV region for exploration, with an improvement of one-two orders of magnitude in sensitivity compared to the current state of the art, much of which was derived from the COMPTEL instrument more than two decades ago. e-ASTROGAM will also achieve a spectacular improvement in terms of source localization (Fig. 1) and energy resolution, and will allow to measure the contribution to the radiation of the Universe in an unknown range (Fig. 2). At higher energies, reaching to GeV and beyond, the sensitivity of e-ASTROGAM will reveal the transition from nuclear processes to those involving electro- and hydro-dynamical, magnetic and gravitational interactions.

Figure 1

Figure 1: An example of the capability of e-ASTROGAM to transform our knowledge of the MeV-GeV sky. The upper left figure shows the 1-30 MeV sky as observed by COMPTEL in the 1990s; the lower shows the simulated Cygnus region in the 1-30 MeV energy region from e-ASTROGAM. On the right, comparison between the view of the Cygnus region by Fermi in 8 years (top) and that by e-ASTROGAM in one year of effective exposure (bottom) between 400 MeV and 800 MeV.

An important characteristic of e-ASTROGAM is its ability to measure polarization in the MeV range, which is afforded by Compton interactions in the detector. Polarization encodes information about the geometry of magnetic fields and adds a new observational pillar, in addition to the temporal and spectral, through which fundamental processes governing the MeV emission can be determined. The addition of polarimetric information will be crucial for a variety of investigations, including accreting BH systems, magnetic field structures in jets, and the emission mechanisms of GRBs. Polarization will provide definitive insight into the presence of hadrons in extragalactic jets and the origin of ultra-high-energy cosmic rays. In the following sections, the core science questions to be addressed by e-ASTROGAM are presented. The requirements that flow from the scientific objectives, and that drive the instrument design, are presented in Section 3.

Figure 2

Figure 2: Compilation of the measurements of the total extragalactic gamma-ray intensity between 1 keV and 820 GeV, with different components from current models; the contribution from MeV blazars is largely unknown. The semi-transparent band indicates the energy region in which e-ASTROGAM will dramatically improve on present knowledge.