  The variability of submillimeter emission provides a useful tool to probe the
accretion physics in low-luminosity active galactic nuclei. We accumulate four
years of observations using Submillimeter Array for Centaurus A, NGC 4374, NGC
4278, and NGC 5077 and one year of observations for NGC 4552 and NGC 4579. All
sources are variable. We measure the characteristic timescale at which the
variability is saturated by modeling these sources' light curve as a damped
random walk. We detect a timescale for all the sources except NGC 4552. The
detected timescales are comparable to the orbital timescale at the event
horizon scale for most sources. Combined with previous studies, we show a
correlation between the the timescale and the black hole mass over three orders
of magnitude. This discovery suggests the sub-mm emission is optically thin
with the emission originating from the event horizon. The mass scaling
relationship further suggests that a group of radio sources with a broadband
spectrum that peaks at submillimeter wavelengths have similar inner accretion
physics. Sources that follow this relationship may be good targets for
high-resolution imaging with the Event Horizon Telescope.

High Energy Astrophysical Phenomena;Astrophysics of Galaxies