The multiple-point fringe-rate method of mapping spectral-line VLBI sources with application to H2O masers in W3-IRS5 and W3/OH/

Abstract

VLBI observations of astrophysical maser spatial structures consisting of a small number of well separated, compact features at each frequency produce relative-fringe spectra having multiple peaks. Giuffrida (1977) has shown that these peaks can be used to produce maps while avoiding some of the worst calibration difficulties of aperture synthesis. The multiple-point fringe-rate method used to map the H2O maser emission in W3-IRS5 and W3(OH) is described, along with some of its advantages and limitations. It is concluded that the additional details seen in these maps, by comparison to previously published observations, make division of the sources into centers of activity difficult.