Megaelectronvolt astronomy with HERMES: spectroscopy, polarimetry, and synergies with fast Cherenkov arrays

Sensitive observations of energetic space phenomena in the 0.3 - 10 MeV range are needed to investigate a number of hot issues of modern astrophysics and cosmology. With the advanced technology of thick double sided silicon strip detectors (Si-DSSDs) it is possible to construct a space-borne gamma-ray telescope with sensitivity 30-100 times better than that of CGRO/COMPTEL. Synergies of MeV range observations from space with GeV-TeV range spectra to be obtained with future fast and sensitive imaging atmospheric Cherenkov arrays (CTA, TAIGA-IACT, ALEGRO) would allow one to reveal complex mechanisms of energy conversion in transient gamma-ray objects, in particular, sources of gamma-ray bursts. In this short note we briefly outline most demanding objectives of 0.3 - 10 MeV range astronomy and present a schematic view of HERMES gamma-ray spectrometer being developed at the Ioffe Institute.