  X-ray timing observations of accreting stellar mass black holes have shown
that they can produce signals with such short time scales that we must be
probing very close to the innermost stable circular orbit that is predicted by
the theory of General Relativity (GR). These signals are quasi-periodic
oscillations (QPOs), and both the high-frequency variety (HFQPOs, which have
frequencies in the 40-450 Hz range) as well as the 0.1-10 Hz low-frequency type
have the potential to provide tests of GR in the strong field limit. An
important step on the path to GR tests is to constrain the physical black hole
properties, and the straightforward frequency measurements that are possible
with X-ray timing may provide one of the cleanest measurements of black hole
spins. While current X-ray satellites have uncovered these phenomenona, the
HFQPOs are weak signals, and future X-ray timing missions with larger effective
area are required for testing the candidate theoretical QPO mechanisms. Another
main goal in the study of accreting black holes is to understand the production
of relativistic jets. Here, we have also made progress during the past decade
by finding clear connections between the radio emission that traces the
strength of the jet and the properties of the X-ray emission. With new radio
capabilities just coming on-line, continuing detailed X-ray studies of
accreting black holes is crucial for continuing to make progress.
