Radio flares and plasmon size in Cygnus X-3

We have observed a number of minor radio flares in Cyg X-3 using the MERLIN array. Photometric observations show the system to be highly active with m ultiple flares on hourly time-scales over the one month observing programme . Analysis of the power spectrum of the source show no persistent periodici ties in these data, and no evidence of the 4.8-h orbital period. An upper l imit of 15 mJy can be placed on the amplitude of any sinusoidal variation o f source flux at the orbital period. The brightness temperature of a flare is typically T-b greater than or equal to 10(9)-10(10) K, with a number of small flares of 5-min duration having brightness temperatures of T-b greate r than or equal to few x 10(11) K. For such a change in flux to occur withi n a typical 10-min time-scale, the radiation must originate from plasmons w ith a size less than or equal to1.22 au. This emission is unlikely to origi nate close to the centre of the system as both the jets and compact object are buried deep within an optically thick stellar wind. Assuming a spherica lly symmetric wind, plasmons would become visible at distances similar to 1 3 au from the core.