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XEUS has been recently selected by ESA for an assessment study. XEUS is a large mission candidate for the
Cosmic Vision program, aiming for a launch date as early as 2018. XEUS is a follow-on to ESA's Cornerstone
X-Ray Spectroscopy Mission (XMM-Newton). It will be placed in a halo orbit at L2, by a single Ariane 5 ECA,
and comprises two spacecrafts. The Silicon pore optics assembly of XEUS is contained in the mirror spacecraft
while the focal plane instruments are contained in the detector spacecraft, which is maintained at the focus of the
mirror by formation flying. The main requirements for XEUS are to provide a focused beam of X-rays with an
effective aperture of 5 m2 at 1 keV, 2 m2 at 7 keV, a spatial resolution better than 5 arcsec, a spectral resolution
ranging from 2 to 6 eV in the 0.1-8 keV energy band, a total energy bandpass of 0.1-40 keV, ultra-fast timing,
and finally polarimetric capabilities. The High Time Resolution Spectrometer (HTRS) is one of the five focal
plane instruments, which comprises also a wide field imager, a hard X-ray imager, a cryogenic spectrometer,
and a polarimeter. The HTRS is unique in its ability to cope with extremely high count rates (up to 2 Mcts/s),
while providing sub-millisecond time resolution and good (CCD like) energy resolution. In this paper, we focus
on the specific scientific objectives to be pursued with the HTRS: they are all centered around the key theme
"Matter under extreme conditions" of the Cosmic Vision science program. We demonstrate the potential of the HTRS observations to probe strong gravity and matter at supra-nuclear densities. We conclude this paper by
describing the current implementation of the HTRS in the XEUS focal plane.
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