  With the opening of the X and gamma--ray windows in the sixties, thanks to to
sounding rockets and satellite-borne instruments, extremely energetic and
violent phenomena were discovered and subsequently found to be ubiquitous in
the Universe. Observations in the high energy domain are fundamental for
understanding how matter is organized and behaves around black holes;
unravelling how these extreme objects influence their environments on a very
large scale; and finding the still elusive obscured massive objects in the
centre of galaxies. Other major problems in contemporary astrophysics, such as
the understanding of acceleration processes at shocks of all sizes (those of
pulsar wind nebulae, supernova remnants, but also at larger scales those of
Active Galactic Nuclei radio lobes) in relation to the origin of cosmic-rays,
or the definitive characterization of the debated non-thermal X-ray energy
content of clusters of galaxies, also requires observations at very high
energies. An observatory type medium mission operating from around 1 keV to
about 600 keV can provide direct insights into these major questions. The
essential characteristics will be coverage of the full energy range by
telescopes featuring a large throughput and arc-second resolution optics,
coupled to a compact focal plane assembly, with excellent imaging resolution
and spectroscopy. In addition, the mission will provide unique polarimetry
measurements in the hard X-ray domain, an important new diagnostic tool at
energies for which the non-thermal processes dominate. The Polarimetric
High-Energy Modular Telescope Observatory (PHEMTO) is designed to have
performance several orders of magnitude better than the present hard X-ray
instruments. This gives to PHEMTO the improvements in scientific performance
needed for a mission in the 2050 era.
