MERLIN imaging of the maser flare in Markarian 348

MERLIN images of Mrk 348 at 22 GHz show H2O maser emission at 0.02-0.11 Jy, within similar to0.8 pc of the nucleus. This is the first direct confirmat ion that molecular material exists close to the Seyfert 2 nucleus. Mrk 348 was observed in 2000 May one month after Falcke et al. first identified the maser in single-dish spectra. The peak maser flux density has increased ab out threefold. The masing region is less than or similar to 0.6 pc in radiu s. The flux density of radio continuum emission from the core has been risi ng for about 2 yr. The maser-core separation is barely resolved, but at the 3 sigma significance level maser and core are not coincident along the lin e 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 exp lained as arising from high-level nuclear activity through a common excitat ion mechanism, although direct maser amplification of the core by masers tr acing a Keplerian disc is not completely ruled out.