With its ability to look at bright galactic X-ray sources with
sub-millisecond time resolution, the Rossi X-ray Timing Explorer (RXTE)
discovered that the X-ray emission from accreting compact stars shows
quasi-periodic oscillations on the dynamical timescales of the strong field
region. RXTE showed also that waveform fitting of the oscillations resulting
from hot spots at the surface of rapidly rotating neutron stars constrain their
masses and radii. These two breakthroughs suddenly opened up a new window on
fundamental physics, by providing new insights on strong gravity and dense
matter. Building upon the RXTE legacy, in the Cosmic Vision exercise, testing
General Relativity in the strong field limit and constraining the equation of
state of dense matter were recognized recently as key goals to be pursued in
the ESA science program for the years 2015-2025. This in turn identified the
need for a large (10 m2 class) aperture X-ray observatory. In recognition of
this need, the XEUS mission concept which has evolved into a single launch L2
formation flying mission will have a fast timing instrument in the focal plane.
In this paper I will outline the unique science that will be addressed with
fast X-ray timing on XEUS.Comment: 12 pages, 6 figures, COSPAR Colloquium "Spectra & Timing of Compact
X-ray Binaries", January 17-20, 2005, Mumbai, India. Advances in Space
Research, 2006, in pres