A Space-based All-sky MeV gamma-ray Survey with the Electron Tracking Compton Camera

A sensitive survey of the MeV gamma-ray skyis needed to understand important astrophysical problems such as gamma-ray burstsin the early universe, progenitors of Type Ia supernovae, and the nature of dark matter. However, the study has not progressed remarkably since the limited survey by COMPTEL onboardCGRO in the 1990s. Tanimori et al. have developed a Compton camera that tracks the trajectory of each recoil electron in addition to the information obtained by the conventional Compton cameras, leading to superior imaging. This Electron Tracking Compton Camera (ETCC) facilitates accurate reconstruction of the incoming direction of each MeV photon from a wide skyat ~degree angular resolutionand with minimized particle background using trajectory information. The latest ETCC model, SMILE-2+, made successful astronomical observations during a day balloon flight in 2018 April and detected diffuse continuum and 511 keV annihilation line emission from the Galactic Centerregion at a high significance in ~2.5 hours. We believe that MeV observations from space with upgraded ETCCs will dramatically improve our knowledge of the MeV universe. We advocate for a space-based all- skysurvey mission with multiple ETCCs onboardand detail its expected benefits.