VSOP Observations of a Sub-Parsec Accretion Disk

The physical conditions in the inner parsec of accretion disks be-lieved to orbit the central black holes in active galactic nuclei can be probed by imaging the absorption of background radio emis-sion by ionized gas in the disk. We report high angular reso-lution observations of the nearby galaxy NGC 4261 which show evidence for free-free absorption by a thin, nearly edge-on disk at several frequencies. The angular width, and probably the depth, of the absorption appears to increase with decreasing frequency, as expected. Because free-free absorption is much larger at lower frequencies, the longest possible baselines are needed to provide adequate angular resolution; observing at higher frequencies to improve resolution will not help.