  Parsec-scale VLBA imaging of five Seyfert galaxies with flat-spectrum radio
nuclei was conducted to determine whether the flat spectrum represents thermal
emission from the accretion disk/obscuring torus or nonthermal, synchrotron
self-absorbed emission. Four of the five show emission consistent with
synchrotron self-absorption, with intrinsic sizes ~0.05-0.2 pc (or 10^4
gravitational radii for a 10^8 Msun black hole for the smallest). In contrast,
NGC 4388, which was detected with MERLIN but not the VLBA, shows thermal
emission with similar properties to that detected in NGC 1068. It is notable
that the two Seyfert galaxies with detected thermal nuclear radio emission both
have large X-ray absorbing columns, suggesting that columns in excess of
\~10^{24} cm^{-2} are needed for such disks to be detectable.
