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F. K. Lamb (UIUC)
X-ray timing observations have proved repeatedly to be a uniquely powerful tool for uncovering the nature of compact systems. They provided the first compelling evidence that most X-ray stars are in close binary systems, that many are neutron stars, and that they are accreting. They proved the most successful way to identify black hole candidates, gave the first estimates of the magnetic fields of X-ray stars, and revealed the existence of thermonuclear flashes on neutron stars. More recently they revealed the time development of X-ray bursts, the existence of coherent orbital motion deep in the strong gravitational fields of neutron stars and black holes, and evidence for the existence of magnetars. Timing is such a powerful tool because it can be used to study repetitive, global motions on the natural timescales of a system. Such motions are easier to relate to theory than other observables and can be measured with high precision.
A new timing mission will make possible major advances in our understanding of strong gravitational fields and compact objects. Only now, with RXTE, are we {\em beginning\/} to be able to study X-ray variations on the natural, dynamical timescales of neutron stars and black holes. For the first time we have detected such variations, but we cannot yet study them in detail in the way we routinely study, e.g., the waveforms of pulsars. A new timing mission that will make possible detailed study of the X-ray variations produced by gas moving in the strongly curved spacetime near the surfaces of neutron stars and the event horizons of black holes will provide powerful direct evidence about the character of strong-field gravity and the properties of ultradense matter, two of the most fundamental outstanding questions in modern astrophysics. This research was supported in part by the NSF and NASA.