VLBA Observations and Effelsberg Monitoring of a Flaring Water Maser in Mrk 348

We report on single-dish monitoring as well as extremely high angular resolution observations of the flaring H2O maser in the Seyfert 2 galaxy Mrk 348. The H2O line is redshifted by ~130 km/s with respect to the systemic velocity, is very broad, with a FWHM of 130 km/s, and has no detectable high velocity components within 1500 km/s on either side of the strong line. Monitoring observations made with the Effelsberg 100m telescope show that the maser varies significantly on timescales as short as one day. VLBA observations indicate that the maser emission arises entirely from a region less than 0.25 pc in extent, located toward a continuum component thought to be associated with the receding jet.