  We present theoretical models of X-ray variability attributable to orbital
signatures from an accretion disk including emission region size,
quasi-periodic oscillations (QPOs) and its quality factor $Q$, and the
emergence of a break frequency in the power spectral density shape. We find a
fractional variability amplitude of $F_{var}\propto M^{-0.4}_{\bullet}$. We
conduct a time series analysis on X-ray light curves ($0.3-10$ keV) of a sample
of active galactic nuclei (AGNs). A statistically significant bend frequency is
inferred in 9 of 58 light curves (16 %) from 3 AGNs for which the break
timescale is consistent with the reported BH spin but not with the reported BH
mass. Upper limits of $2.85 \times 10^7 M_\odot$ in NGC 4051, $8.02 \times 10^7
M_\odot$ in MRK 766 and $4.68 \times 10^7 M_\odot$ in MCG-6-30-15 are inferred
for maximally spinning BHs. For REJ 1034+396, where a QPO at 3733 s was
reported, we obtain an emission region size of $(6 - 6.5) M$ and a BH spin
$a\lesssim$ 0.08. The relativistic inner region of a thin disk, dominated by
radiation pressure and electron scattering is likely to host the orbital
features as the simulated $Q$ ranges from $6.3 \times 10^{-2}$ to $4.25 \times
10^6$, containing the observed $Q$. The derived value of $Q \sim$ 32 for REJ
1034+396 therefore suggests that the AGN hosts a thin disk.
