MERLIN imaging of the maser flare in Markarian 348

MERLIN images of Mrk 348 at 22 GHz show H 2 O maser emission at 0.02 – 0.11 Jy, within ∼ 0.8 pc of the nucleus. This is the first direct confirmation that molecular material exists close to the Seyfert 2 nucleus. Mrk 348 was observed in 2000 May one month after Falcke et al. (2000) first identified the maser in single-dish spectra. The peak maser flux density has increased about threefold. The masing region is < ∼ 0.6 pc in radius. The flux density of radio continuum emission from the core has been rising for about 2 years. The maser-core separation is barely resolved but at the 3σ significance level they are not coincident along the line of sight. The masers lie in the direction of the northern radio lobes and probably emanate from material shocked by a jet with velocity close to c. The correlation between the radio continuum increase and maser flare is explained as arising from high level nuclear activity through a common excitation mechanism although direct maser amplification of the core by masers tracing a Keplerian disc is not completely ruled out.