The Megamaser Cosmology Project. VII. Investigating disk physics using
  spectral monitoring observations
  We use single-dish radio spectra of known 22 GHz H$_2$O megamasers, primarily
gathered from the large dataset observed by the Megamaser Cosmology Project, to
identify Keplerian accretion disks and to investigate several aspects of the
disk physics. We test a mechanism for maser excitation proposed by Maoz & McKee
(1998), whereby population inversion arises in gas behind spiral shocks
traveling through the disk. Though the flux of redshifted features is larger on
average than that of blueshifted features, in support of the model, the
high-velocity features show none of the predicted systematic velocity drifts.
We find rapid intra-day variability in the maser spectrum of ESO 558-G009 that
is likely the result of interstellar scintillation, for which we favor a nearby
($D \approx 70$ pc) scattering screen. In a search for reverberation in six
well-sampled sources, we find that any radially-propagating signal must be
contributing $\lesssim$10% of the total variability. We also set limits on the
magnetic field strengths in seven sources, using strong flaring events to check
for the presence of Zeeman splitting. These limits are typically 200--300 mG
($1\sigma$), but our most stringent limits reach down to 73 mG for the galaxy
NGC 1194.
