Context : The standard unified scheme of active galactic nuclei requires the presence of high column densities of gas and dust potentially obscuring the central engine .
So far , few direct subarcsecond resolution studies of this material have been performed toward radio galaxies .

Aims : The goal of this paper is to elucidate the nuclear environment of the prototypical X-shaped Fanaroff-Riley type II radio galaxy 3C 403 , the only powerful radio galaxy known to host an H _ { 2 } O megamaser .

Methods : Very Large Array A-array and single-dish Green Bank and Effelsberg 1.3 cm measurements were performed to locate and monitor the water maser emission .
Very Long Baseline Interferometry 6 cm continuum observations were taken to analyze the spatial structure of the nuclear environment at even smaller scales , while the CO J =1–0 and 2–1 transitions were observed with the IRAM 30-m telescope to search for thermal emission from a spatially extended , moderately dense gas component .

Results : Positions of the H _ { 2 } O maser features and the continuum emission from the core coincide within 5 mas ( 5.5 pc ) .
Intensities of the two main maser components with ( isotropic ) luminosities sometimes surpassing 1000 L { { } _ { \odot } } appear to be anti-correlated , with typical timescales for strong variations of one year .
If the variations are intrinsic to the cloud ( s ) , the implied angular source size would be \la 0.3 mas and the brightness temperature \ga 5 \times 10 { { } ^ { 8 } } K. The VLBI continuum observations support a scenario where a nuclear core , represented by the dominant central radio continuum component , is accompanied by a jet and counterjet , directed toward the western and eastern large scale lobes of the galaxy .
CO remains undetected , providing a maximum scale size of \sim 50 pc \times ( 500 K/ T _ { b } ) ^ { 1 / 2 } , with T _ { b } denoting the brightness temperature of the CO J =1–0 line .
Possible scenarios that could produce the observed maser emission are outlined .
Adopting a mass of several 10 ^ { 8 } for the nuclear engine , the observed maser features can only be interpreted in terms of an accretion disk as in NGC 4258 , if they solely represent the systemic velocity components .
The receding and approaching parts of the putative maser disk are , however , not seen and a secular velocity drift of the observed features is not ( yet ) apparent .
Most likely , the two main maser components mark shocked molecular gas interacting with the nuclear jets .
The X-shaped morphology of the radio galaxy may point at a binary nuclear engine .
This possibility , greatly complicating the nuclear environment of 3C 403 , should motivate a number of worthwhile follow-up studies .

Conclusions :