2000ApJ...530..233W

Nearly simultaneous VLBA observations of 3C 84, the radio source associated with NGC 1275, have been made at multiple frequencies to study the free-free absorption of the northern, or ``counterjet'', feature found by Walker, Romney, & Benson and by Vermeulen, Readhead, & Backer. Our observations confirm that the spectra are consistent with free-free absorption and eliminate the possibility that the earlier result was an effect of variability. The northern feature is well resolved spatially, so images have been made showing the distribution of the absorption over a region of about 1.5 pc on a side, beginning about 1.5 pc from the presumed location of the central object. That distribution is dominated by a strong decrease with radial distance. The magnitude of the absorption near 2.5 pc projected distance from the central object is consistent with a 10⁴ K gas with an emission measure of about 5x10⁸ pc.cm^–6. The geometry is consistent with absorption by ionized gas associated with an accretion disk. The data provide firm constraints for models of the outer regions of accretion disks and, perhaps, associated winds.