We report the detection of a 22 GHz water maser line in the nearest ( D \sim 3.8 Mpc ) radio galaxy Centaurus A using the Australia Telescope Compact Array ( ATCA ) .
The line is centered at a velocity of \sim 960 km s ^ { -1 } , which is redshifted by about 400 km s ^ { -1 } from the systemic velocity .
Such an offset , as well as the width of \sim 120 km s ^ { -1 } , could be consistent with either a nuclear maser arising from an accretion disk of the central supermassive black hole , or for a jet maser that is emitted from the material that is shocked near the base of the jet in Centaurus A .
The best spatial resolution of our ATCA data constrains the origin of the maser feature within < 3 pc from the supermassive black hole .
The maser exhibits a luminosity of \sim 1 L _ { \odot } , which classifies it as a kilomaser , and appears to be variable on timescales of months .
A kilomaser can also be emitted by shocked gas in star forming regions .
Given the small projected distance from the core , the large offset from systemic velocity , as well as the smoothness of the line feature , we conclude that a jet maser line emitted by shocked gas around the base of the AGN is the most likely explanation .
For this scenario we can infer a minimum density of the radio jet of \gtrsim 10 cm ^ { -3 } , which indicates substantial mass entrainment of surrounding gas into the propagating jet material .