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The Rossi X-ray Timing Explorer (RXTE) has demonstrated the power
of observations in the time domain in the study of X-ray binaries.
The dynamical variation of the X-ray emission from accreting
neutron stars and stellar mass black holes is a powerful probe of
their strong gravitational fields. At the same time, oscillations
at the neutron star spin frequency during X-ray bursts have been
used to set important constraints on the mass and radius of
neutron stars. The X-ray Evolving Universe Spectroscopy mission
(XEUS), the potential follow-on mission to XMM-Newton, will have
a mirror aperture more than ten times larger than the effective
area of the RXTE proportional counter array (RXTE/PCA). Combined
with a small dedicated fast X-ray timing detector in the focal
plane (XTRA: XEUS Timing for Relativistic Astrophysics), this
collecting area will provide a leap in timing sensitivity of more
than one order of magnitude over the RXTE/PCA for bright sources,
and will open a brand new window on faint X-ray sources, owing to
the negligible detector background. Furthermore, the use of
advanced Silicon drift chambers will improve the energy resolution by a factor of ~ 6 over the RXTE/PCA. The spectroscopic diagnostics of the strong field region, such as the gravitational redshift and the relativistic broadening of the Iron line, will become exploitable simultaneous with the fast timing. XTRA will provide unique opportunities, enabling for example the testing of general relativity in the strong gravity field regime and investigating with unprecedented accuracy the equation of state of matter at supranuclear density. XTRA when observing bright X-ray sources will record typically several hundred thousand counts per second and careful design will be necessary to ensure that the full potential of these data can be exploited. We describe the proposed implementation of XTRA in the XEUS focal plane using Silicon drift detectors and present the current performance of these devices.
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