  We report dual-frequency very long baseline interferometry (VLBI)
observations at 22 and 43 GHz toward the nucleus of a nearby radio galaxy NGC
4261. In particular, we present a VLBI image of the 22 GHz H2O maser line and
its location in the circumnuclear region of NGC 4261. H2O maser emission is
marginally detected above the three times the rms level within a velocity range
of approximately 2250-2450 km/s, slightly red-shifted with respect to the
systemic velocity. H2O maser emission is located approximately 1 milliarcsecond
(mas) east of the brightest continuum component at 22 GHz, where the continuum
spectrum is optically thick, that is at the free-free absorbed receding jet by
ionized gas. A positional coincidence between H2O maser emission and an ionized
gas disk implies that the H2O maser emission arises from the near side of the
disk, amplifying continuum emission from the background receding jet. If the
disk axis is oriented 64 degree relative to the line of sight, the H2O maser
emission is expected to be at a mean radius of 0.3 pc in the disk. The broad
line width of the H2O maser emission can be attributed to complex kinematics in
the immediate vicinity of the supermassive black hole (SMBH), including ongoing
gas infall onto the SMBH, turbulence, and outflow. This is analogous to the
multi-phase circumnuclear torus model in the nearest radio-loud H2O megamaser
source NGC 1052. An alternative explanation for H2O maser association is the
shock region between the jet and the ambient molecular clouds. However, this
explanation fails to describe the explicit association of H2O maser emission
only with the free-free absorbed receding jet.
