In the shadow of the accretion disk: Higher resolution imaging of the central parsec in NGC 4261

The physical conditions in the inner parsec of accretion disks believed to orbit the central black holes in active galactic nuclei can be probed by im aging the absorption (by ionized gas in the disk) of background emission fr om a radio counterjet. We report high angular resolution VLBI observations of the nearby (similar to 40 Mpc) radio galaxy NGC 4261 that confirm free-f ree absorption of radio emission from a counterjet by a geometrically thin, nearly edge-on disk at 1.6, 4.8, and 8.4 GHz. The angular width and depth of the absorption appears to increase with decreasing frequency, as expecte d. We derive an average electron density of similar to 10(4) cm(-3) at a di sk radius of about 0.2 pc, assuming that the inner disk inclination and ope ning angles are the same as at larger radii. Pressure balance between the t hermal gas and the magnetic field in the disk implies an average field stre ngth of 10(-4) G at a radius of 0.2 pc. These are the closest in free-free absorption measurements to date of the conditions in an extragalactic accre tion disk orbiting a black hole with a well-determined mass. If a standard advection-dominated accretion flow exists in the disk center, then the tran sition between thin and thick disk regions must occur at a radius less than 0.2 pc (4000 Schwarzschild radii).