  In X-ray binaries, the frequencies revealed in X-ray quasi-periodic
oscillations (QPOs) are often interpreted as characteristic frequencies in the
inner accretion disk, though the exact oscillation mechanism is unknown at
present. Broadened Fe K-alpha lines are also excellent diagnostics of the inner
accretion flow, if their broadening is indeed due to inner disk reflection.
Herein, we present two cases where the flux and equivalent width of the Fe
K-alpha emission lines in spectra of the Galactic black hole GRS 1915+105 vary
with the phase of strong 1 Hz and 2 Hz QPOs in the X-ray flux. These results
provide strong evidence that both QPOs and the Fe-alpha lines originate in the
inner disk. If the 1 Hz QPO is only a Keplerian orbital frequency, the QPO
comes from a distance of 84 +/- 26 R_Schw from the black hole; the 2 Hz QPO
implies a radius of 50 +/- 15 R_Schw. At these radii, relativistic shaping of a
disk line is inevitable. Moreover, the link holds in radio-bright and
radio-faint phases, signaling that in systems like GRS 1915+105, the Fe K-alpha
line is a disk line and not a jet line as per SS 433. A particularly
interesting possibility is that a stable warp in the inner disk, e.g. due to
Lense-Thirring precession, may produce the observed QPOs and line modulations.
More broadly, the FeK-QPO link provides an unprecedented mechanism for
revealing the inner accretion flow and relativistic regime in accreting
systems, in that it gives two measures of radius: for a given disk QPO model,
the frequency translates into a specific radius, and relativistic line models
yield radii directly.
