First VLBI Observations of Extragalactic Water Vapor Maser Flares in IC10
Session 75 -- Star Formation II
Oral presentation, Thursday, 2:30-4:00, Zellerbach Playhouse Room

## [75.07] First VLBI Observations of Extragalactic Water Vapor Maser Flares in IC10

A.L.~Argon, L.J.~Greenhill, M.J.~Reid, J.M.~Moran, K.M.~Menten (CfA), C.~Henkel (MPIfR), M.~Inoue (Nobeyama Radio Observatory)

\def\lo {~L$_{\odot}$} The water vapor maser associated with the south$-$eastern nucleus of IC10, as traced in CO emission, has been observed with a six station intercontinental VLBI array over three epochs spanning one year (from Nov 1990 to Nov 1991). The absolute position of the maser and the relative positions of five spectral features were measured. We found that the brightest spectral feature reached an integrated luminosity of $\sim$ 1.7 \lo, assuming isotropic emission of radiation and a distance of 2 Mpc. Single dish observations made in late 1992 and early 1993 suggest that this feature has recently become much stronger, with an integrated luminosity approaching 10 \lo. In addition, we found that one of the features decreased by $\sim$ 60$\%$ in flux density over an interval of 17 hours during our VLBI experiment. Less dramatic but noticeable variations were observed on time scales of about an hour. Separate VLA observations show that the maser is associated with a compact radio continuum source. High luminosity coupled with rapid time variability put important constraints on possible emission mechanisms.