VLBI Observations of Cygnus X-3 during the 1985 October Radio Outburst

Cygnus X-3 underwent a large outburst beginning on 1985 October 2 and reached a peak flux density of 18 Jy at 11.1 cm on October 10. Between October 7 and October 19 we performed seven snapshot observations at a wavelength of 6 cm using telescopes of the European VLBI Network. The source structure may be represented by three components, consistent with a pair of symmetrical jets emanating from a central compact object. The component sizes are consistent with broadening by interstellar scattering to 16±6 mas. Comparison with other data shows that the frequency dependence of the scattered image varies as v-2.07±0.04 and that the scattering toward Cygnus X-3 is enhanced compared with other Galactic sources. We estimate that the emission in the symmetrical components has an apparent proper motion of 4.2 mas day-1 or 0.3c at a distance of 10 kpc. Study of the VLBI structure and total flux density variability yields an upper limit for the distance to Cygnus X-3 of 15 kpc.