Time-sequenced multi-radio frequency observations of Cygnus X-3 in flare

Multifrequency observations from the Very Large Array, Very Long Baseline Array (VLBA), and Owens Valley Radio Observatory Millimeter Array of a major radio outburst of Cygnus X-3 in 2001 September are presented, measuring the evolution of the spectrum of the source over 3 decades in frequency, over a period of 6 days. Following the peak of the flare, as the intensity declines the high-frequency spectrum at frequency nu steepens from nu(-0.4) to nu(-0.6), after which the spectral index remains at this latter terminal value, a trend previously observed but hitherto not satisfactorily explained. VLBA observations, for the first time, track over several days the expansion of a sequence of knots whose initial diameters are similar to8 mas. The light-crossing time within these plasmons is of the same order as the timescale over which the spectrum is observed to evolve. We contend that properly accounting for light-travel time effects in and between plasmons that are initially optically thick but that after expansion become optically thin explains the key features of the spectral evolution, for example, the observed timescale. Using the VLBA images, we have directly measured for the first time the proper motions of individual knots, analysis of which shows a two-sided jet whose axis is precessing. The best-fit jet speed is beta similar to 0.63, and the precession period is similar to5 days, significantly lower than that fitted for a previous flare. Extrapolation of the positions of the knots measured by the VLBA back to zero separation shows this to occur approximately 2.5 days after the detection of the rise in flux density of Cyg X-3.