We monitored the 22 GHz maser line in the lensed quasar MG J0414+0534 at z=2.64 with the 300-m Arecibo telescope for almost two years to detect possible additional maser components and to measure a potential velocity drift of the lines .
The main maser line profile is complex and can be resolved into a number of broad features with line widths of 30-160 km s ^ { -1 } .
A new maser component was tentatively detected in October 2008 at a velocity of +470 km s ^ { -1 } .
After correcting for the estimated lens magnification , we find that the H _ { 2 } O isotropic luminosity of the maser in MG J0414+0534 is \sim 26,000 solar luminosities , making this source the most luminous ever discovered .
Both the main line peak and continuum flux densities are surprisingly stable throughout the period of the observations .
An upper limit on the velocity drift of the main peak of the line has been estimated from our observations and is of the order of 2 km s ^ { -1 } per year .
We discuss the results of the monitoring in terms of the possible nature of the maser emission , associated with an accretion disk or a radio jet .
This is the first time that such a study is performed in a water maser source at high redshift , potentially allowing us to study the parsec-scale environment around a powerful radio source at cosmological distances .