Sub-Parsec-Scale Jet-Driven Water Maser with Possible Gravitational Acceleration in the Radio Galaxy NGC 1052

We report sub-pc-scale observations of the 321-GHz H$_2$O emission line in the radio galaxy NGC 1052. The H$_2$O line emitter size is constrained in $< 0.6$ milliarcsec distributed on the continuum core component. The brightness temperature exceeding $10^6$ K and the intensity variation indicate certain evidence for maser emission. The maser spectrum consists of redshifted and blueshifted velocity components spanning $\sim 400$ km s$^{-1}$, separated by a local minimum around the systemic velocity of the galaxy. Spatial distribution of maser components show velocity gradient along the jet direction, implying that the population-inverted gas is driven by the jets interacting with the molecular torus. We identified significant change of the maser spectra between two sessions separated by 14 days. The maser profile showed a radial velocity drift of $127 \pm 13$ km s$^{-1}$ yr$^{-1}$ implying inward gravitational acceleration at 5000 Schwarzschild radii. The results demonstrate feasibility of future VLBI observations to resolve the jet-torus interacting region.