2007A&A...474..903B

The Galactic microquasar SS433 launches oppositely-directed plasma jets at speeds approximately a quarter of the speed of light along an axis which precesses, tracing out a cone of polar angle 20 degrees. Both the speed and direction of launch of the bolides that comprise the jets exhibit small fluctuations. To present new results on variations in speed and direction of successive bolides over periods of one day or less and the range in speed and direction exhibited by material within individual bolides. To present the expansion rate of bolides revealed by optical line broadening. To integrate these results with earlier data sensitive to fluctuations and symmetry over longer timescales. New high resolution spectra, taken with a 3.6-m telescope almost nightly over 0.4 of a precession cycle, are analysed in terms of fluctuations in the properties of the ejecta, by comparing the redshift data with predictions for small departures from the simple kinematic model. The variations of redshifts of the jets within a day are studied, in addition to the longer term variations of the daily averaged properties. (i) These data exhibit multiple ejections within most 24-h periods and, throughout the duration of the observing campaign, the weighted means of the individual bolides, in both the red jet and the blue jet, clearly exhibit the pronounced nodding known in this system; (ii) we present further evidence for a 13-day periodicity in the jet speed, and show this cannot be dominated by Doppler shifts from orbital motion; (iii) we show the phase of this peak jet speed has shifted by a quarter of a cycle in the last quarter-century; (iv) we show that the two jets ejected by SS433 are highly symmetric on timescales measured thus far; (v) we demonstrate that the anti-correlation between variations in direction and in speed is not an artifact of an assumption of symmetry; (vi) we show that a recently proposed mechanism (Begelman et al., 2006MNRAS.370..399B) for varying the ejection speed and anti-correlating it with polar angle variations is ruled out; (vii) the speed of expansion of the plasma bolides in the jets is approximately 0.0024c. These novel data carry a clear signature of speed variations. They have a simple and natural interpretation in terms of both angular and speed fluctuations which are identical on average in the two jets. They complement archival optical data and recent radio imaging.