AGN accretion disk physics using H2O megamasers

Many accretion disks surrounding supermassive black holes in nearby AGN are observed to host 22 GHz water maser activity. We have analyzed single-dish 22 GHz spectra taken with the GBT to identify 32 such ``Keplerian disk systems,'' which we used to investigate maser excitation and explore the possibility of disk reverberation. Our results do not support a spiral shock model for population inversion in these disks, and we find that any reverberating signal propagating radially outwards from the AGN must constitute <10% of the total observed maser variability. Additionally, we have used ALMA to begin exploring the variety of sub-mm water megamasers that are also predicted, and in the case of the 321 GHz transition found, to be present in these accretion disks. By observing multiple masing transitions within a single system, we can better constrain the physical conditions (e.g., gas temperature and density) in the accretion disk.