VLBI observations of the X-ray binary LS I +61 deg 303

Abstract

VLBI observations of LS I +61 deg 303 revealed a resolved source with an angular diameter of 3.2 mas during a period of low flux density. This observation corresponds to the time of onset of a new radio outburst and several days following the previous outburst. Near peak flux density the sources are unresolved, with an upper limit to the angular diameter of 1.6 mas corresponding to a linear diameter of 3.7 AU. The low source expansion velocities implied by these results argue against the supercritical accretion model as proposed by Taylor and Gregory (1984) in which Roche lobe overflow occurs at periastron of a highly eccentric orbit. A modified SA model is suggested in which the compact companion orbits in a moderately eccentric orbit in the environment of a Be star wind and accretes therefrom at supercritical rates for a fraction of the orbit.