GRAVITAS : General Relativistic Astrophysics VIa Timing And
  Spectroscopy: An ESA M3 mission proposal
  GRAVITAS is an X-ray observatory, designed and optimised to address the ESA
Cosmic Vision theme of "Matter under extreme conditions". It was submitted as a
response to the call for M3 mission proposals. The concept centres around an
X-ray telescope of unprecedented effective area, which will focus radiation
emitted from close to the event horizon of black holes or the surface of
neutron stars. To reveal the nature and behaviour of matter in the most extreme
astrophysical environments, GRAVITAS targets a key feature in the X-ray spectra
of compact objects: the iron Kalpha line at ~6.5 keV. The energy, profile, and
variability of this emission line, and the properties of the surrounding
continuum emission, shaped by General Relativity (GR) effects, provide a unique
probe of gravity in its strong field limit. Among its prime targets are
hundreds of supermassive black holes in bright Active Galactic Nuclei (AGN),
which form the perfect laboratory to help understand the physical processes
behind black hole growth. Accretion plays a fundamental role in the shaping of
galaxies throughout cosmic time, via the process of feedback. Modest
(~sub-arcmin) spatial resolution would deliver the necessary sensitivity to
extend high quality X-ray spectroscopy of AGN to cosmologically-relevant
distances. Closer to home, ultra-high count rate capabilities and
sub-millisecond time resolution enable the study of GR effects and the equation
of state of dense matter in the brightest X-ray binaries in our own Galaxy,
using multiple probes, such as the broad iron line, the shape of the disk
continuum emission, quasi-periodic oscillations, reverberation mapping, and
X-ray burst oscillations. Despite its breakthrough capabilities, all enabling
technologies for GRAVITAS are already in a high state of readiness. It is based
on ultra light-weight X-ray optics and a focal plane detector using silicon
technology.
