The gamma-ray energy range from a few hundred keV to a few hundred MeV has remained largely unexplored since the pioneering but limited observations by COMPTEL on CGRO (1991-2000). Fundamental astrophysics questions can be addressed by a mission in the MeV range, from astrophysical jets and extreme physics of compact objects to a large population of unidentified objects.
To address these questions, we are developing AMEGO: All-sky Medium Energy Gamma-ray Observatory, a NASA Probe class mission, to investigate the energy range from 300 keV to >10 GeV with good energy (ranging from <1% at the low end to ~10% at the high end) and angular resolution (from 2 to 6 degrees depending on energy) and with sensitivity approaching a factor of 20-50 better than previous instruments. Measurements at these energies are challenging, mainly due to the specifics of photon detection: Compton scattering and pair production compete. These two interaction processes require different approaches in both detection and data analysis, and consequently in the instrument concept. AMEGO will be capable of measuring both Compton-scattering events at lower energies and pair-production events at higher energies. Also, AMEGO will have sensitivity to linear polarization of detected radiation at a level of 20% minimum detectable polarization from a source 1% of the Crab, observed for 106 s. AMEGO will operate mainly in scanning (discovery) mode with field-of-view 2.5 sr (20% of the sky at any time).