Asymmetry in the Spectrum of High-Velocity H2O Maser Emission Features
  in Active Galactic Nuclei
  We suggest a mechanism for the amplification of high-velocity water-vapor
maser emission features from the central regions of active galactic nuclei. The
model of an emitting accretion disk is considered. The high-velocity emission
features originate in the right and left wings of the Keplerian disk. The
hyperfine splitting of the signal levels leads to an asymmetry in the spectral
profile of the water vapor maser line at a frequency of 22.235 GHz. We show
that the gain profile asymmetry must lead to an enhanced brightness of the
blueshifted high-velocity emission features compared to the redshifted ones.
Such a situation is observed in the source UGC 3789.
