THE EVOLUTION OF OUTBURSTS IN CYGNUS-X-3 AT 2.25 AND 8.3 GHZ

Detailed observations of a long outburst in the X-ray binary Cygnus X-3 at 2.25 and 8.3 GHz in early 1994 are presented. The source has been monitored at dual radio frequencies on a daily basis with the Green Bank Interferometer since 1982. This outburst was predicted based on the behavior of the quiescent flux densities prior to the major flare. The data show that the 1994 February 20 flare occurred just one day after 19 days of very low quiescent flux densities and exhibited higher opacity at 2 GHz than usual for Cygnus X-3 flares. Later flares in the 1994 February-March outburst sequence displayed progressively less opacity as determined from the observed ratios of peak flux densities at the two frequencies, the lags between the peaks, and the spectral index. Examination of the evolution of spectral index during the 1994 February outburst, compared to previous outbursts in the history of Cygnus X-3, shows that the trend toward optically thin flares begins with the onset of outburst activity immediately following the very low quiescent flux densities and may relate to the very low flux densities. This behavior can be interpreted either as evidence for a declining opacity from thermal gas in the binary system or for evolution of the exponential lateral expansion rate of the jet during the series of flares in the outburst. © 1995 American Astronomical Society.