MERLIN imaging of the maser flare in Markarian 348

Abstract

MERLIN images of Mrk 348 at 22 GHz show water maser emission at 0.02 - 0.11 Jy, within approximately 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 3 sigma 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.