VLBI OBSERVATIONS OF THE X-RAY BINARY LS-I + 61-DEGREES-303

LS 1 +61-degrees 303 has been observed with the European VLBI Network (EVN) in two sessions during a radio outburst in 1987. In the first session of observations the source had a flux density of 46 +/- 3 mJy and a Gaussian fit to the visibility amplitudes yields an angular size of 3.2 +/- 0.9 mas. In the second session, 5 days later, the flux had risen to near peak flux density of 240 +/- 12 mJy. The increased radio emission is of smaller angular size (1.6 +/- 1.2 mas). The upper limit on the linear size of the radio source at peak flux density is 3.7 AU and the brightness temperature, T(b) > 7 x 10(9) K. The limit on the linear size at peak flux yields an upper limit for the average expansion velocity of the outburst ejecta over the 5 days between observations of 640 km s-1. If the larger angular size measured during the first observing session is attributed to expansion of material ejected from the previous orbital cycle, an expansion velocity of 205 km s-1 is obtained. Such a low velocity is consistent with a model for LS 1 +61-degrees-303 that invokes a young pulsar as the source of relativistic particles but is difficult to reconcile with the simple version of the supercritical accretion model as proposed by Taylor and Gregory in 1984. A revised supercritical accretion model, consistent with the new results, is discussed. In the revised model a 1.4 M. companion accretes from the dense equatorial wind of the Be star primary. Calculations of the accretion rate as a function of orbital phase yield a phase interval of supercritical accretion that is similar to the interval of energetic particle injection as inferred from the radio light curve.