STRUCTURE AND MAGNETIC FIELDS IN THE PRECESSING JET SYSTEM SS 433. III. EVOLUTION OF THE INTRINSIC BRIGHTNESS OF THE JETS FROM A DEEP MULTI-EPOCH VERY LARGE ARRAY CAMPAIGN

We present a sequence of five deep observations of SS 433 made over the summer of 2007 using the Very Large Array in the A configuration at 5 and 8 GHz. In this paper, we study the brightness profiles of the jets and their time evolution. We also examine the spectral index distribution in the source. We find (as previously reported from the analysis of a single earlier image) that the profiles of the east and west jets are remarkably similar if projection and Doppler beaming are taken into account. The sequence of five images allows us to disentangle the evolution of brightness of individual pieces of jet from the variations of jet power originating at the core. We find that the brightness of each piece of the jet fades as an exponential function of age (or distance from the core), , where Delta *t is the age at emission and Delta *t' = 55.9 ? 1.7 days. This evolutionary model describes both the east and west jets equally well. There is also significant variation (by a factor of at least five) in jet power with birth epoch, with the east and west jets varying in synchrony. The lack of deceleration between the scale of the optical Balmer line emission (1015 cm) and that of the radio emission (1017 cm) requires that the jet material is much denser than its surroundings. We find that the density ratio must exceed 300:1.