We calculate upper bounds on the inner radii of geometrically thin accretion disks in galactic blackhole systems by relating their rapid variability properties to those of neutron stars. We infer that the inner disk radii do not exhibit large excursions between different spectral states, in contrast with the concept that the disk retreats significantly during the soft-to-hard state transition. We find that, in the hard state, the accretion disks extend down to radii ∼ < 6 − 25 GM/c 2 and discuss the implications of our results for models of black-hole X-ray spectra. Subject headings: accretion, accretion disks — black hole physics — X-rays: star
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