  There is some evidence, though yet unconfirmed, that Sagittarius A*--the
supermassive black hole at the Galactic center--emits its radio waves modulated
with a ~100-day period. What is intriguing about this apparent
quasi-periodicity is that, though the amplitude of the modulation increases
with decreasing wavelength (from 3.6 to 1.3 cm), the quasi-period itself does
not seem to depend on the frequency of the radiation. It is difficult to
imagine how a binary companion, were that the cause of this modulation, could
have escaped detection until now. Instead, it has been suggested that the
spin-induced precession of a disk surrounding a slowly rotating black hole
could have the right period to account for this behavior. In this paper, we
examine how Sagittarius A*'s light curve could be modulated by this mechanism.
We demonstrate that the partial occultation of a nonthermal halo by a compact,
radio-opaque disk does indeed produce the observed frequency-dependent
amplitude. This appears to be in line with other observational arguments
suggesting that Sagittarius A*'s mm/sub-mm spectrum is produced by a ~10
Schwarzschild-radius disk, whereas its cm-waves originate from a nonthermal
particle distribution in a halo extending out to over 20 Schwarzschild radii.
Interestingly, this model suggests that the observed period corresponds to half
the precession period and that a non-axisymmetric disk could produce a second
period roughly twice as long as the first.
