The angular structure and intraday variability of a water vapor maser in IC 10

The water vapor maser associated with the southeastern nucleus of the irregular galaxy IC 10 has been observed with a six-station intercontinental VLBI array over 3 epochs spanning the period 1990 November to 1991 November. The absolute position of the maser and the relative positions of six spectral features were measured. These features are all unresolved and smaller than ~10^15^ cm (based on a distance of 1.25 Mpc) and have brightness temperatures greater than 3 x 10^13^ K. The angular distribution of features indicates a region of activity ~0.06 pc across. Features of disparate velocity are also clustered on a scale of 10^15^ cm, similar to Galactic masers. We found that the brightest spectral feature reached an integrated luminosity of ~0.4 L_sun_, assuming isotropic emission of radiation. Single-dish observations made in late 1922 and early 1993 suggest that this feature has recently become much stronger, with an integrated luminosity approaching 3 L_sun_. In addition, we found that one of the features decreased by ~60% in flux density over an interval of 17 hr during our VLBI experiment. This is the first intraday variation of an H_2_O maser witnessed at many telescopes simultaneously. Systematic, although less dramatic variations were observed on timescales of ~1 hr. Separate VLA observations show that the maser is coincident with a radio continuum source that is smaller than 0.6 pc. We discuss the constraints on possible emission mechanisms imposed by the high luminosity and rapid time variability. We also show that the intraday variation could have been caused by an extreme scattering event due to a cloud in the Galactic ISM.