Testing the Linear Relationship between Black Hole Mass and Variability Timescale in Low-luminosity AGNs at Submillimeter Wavelengths

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 the 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 curves 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 timescale and the black hole mass over 3 orders of magnitude. This discovery suggests the submillimeter 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.