Possible Evidence for Truncated Thin Disks in the Low-Luminosity Active Galactic Nuclei M81 and NGC 4579

Abstract

M81 and NGC 4579 are two of the few low-luminosity active galactic nuclei which have an estimated mass for the central black hole, detected hard X-ray emission, and detected optical/UV emission. In contrast to the canonical ``big blue bump,'' both have optical/UV spectra which decrease with increasing frequency in a $\nu L_\nu$ plot. Barring significant reddening by dust and/or large errors in the black hole mass estimates, the optical/UV spectra of these systems require that the inner edge of a geometrically thin, optically thick, accretion disk lies at roughly 100 Schwarzschild radii. The observed X-ray radiation can be explained by an optically thin, two temperature, advection-dominated accretion flow at smaller radii.