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 similar to 130 km s(-1) with respect to the systemic velocity, is very broad, with an FWHM of 130 km s(-1), and has no detectable high-velocity components within 1500 km s(-1) on either side of the strong line. Monitoring observations made with the Effelsberg 100 m 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.