VLBI observations of expansion in Cygnus X-3

Abstract

We have made VLBI observations at 1.3 cm wavelength of two flares in 1985 February. Fitting these data with simple dynamic models, we arrive at the following conclusions. The times at which flares began are consistent with our previously published period of 4.95 hr (significantly offset from the orbital period of 4.79 hr). The data are neither consistent with simple, static source structure nor simple, circularly symmetric structure. The data are consistent with a source expanding linearly in time from zero size at the flare beginning along a north-south axis (and possibly also expanding at a smaller rate along the transverse axis). This expanding structure is convolved with a fixed-size Gaussian (FWHM 0.68 +/- 0.08 mas) consistent with an extrapolation of longer wavelength measurements of interstellar scattering. Interpreting the elongation as the bulk motion of a double-sided jet, we estimate the projected bulk velocity lies between 0.16c and 0.31c where both statistical and systematic effects are fully accounted for, a range that includes the actual bulk velocity of SS 433 (0.26c). We estimate the expansion transverse to the jet axis to be 0.13 +/- 0.02c, in quantitative agreement with our previously predicted range of 0.05c to 0.25c.