Radio properties of SS 433

Abstract

The object SS ~33 is a comparitively intense radio star (L~I032 ergs -I) exhibiting variable non-thermal radio emission (Seaquist et al. 1979). The centroid of the radio emission coincides with the optical posi$ion to within 0~3, which is withinthe measurement error (Kaplan et al. 1980). Fi6~zre 1 shows the radio spectrum obtained with the VLA* and the Culgoora Radioheliograph on 1979 July 20 (Seaquist et al. 1980). At the higher frequencies the spectral index is~-0.6 (~), corresponding to a power law particle energy spectrum with an index of -2.2. There is a distinct turnover or flattening in the spectrum ate0.3 GHz~ which may be caused by one or more of several mechanisms. The possible mechanisms include the effects of a low energy cutoff in the radiating particle spectrum~ the Razin Effect~ synchrotron self-absorption~ and free-free absorption by ionized gas. An attractive possibility is the last one since there is abundant evidence from optical emission lines and infrared studies that the object is surrounded by an extensive cloud of ionized gas. This gas may result from a stellar wind with a mass loss rate of~10 -5 M@ yr -I or more and a velocity of 1000-2000 kms -I (van den Heuvel, 0strikerp and Petterson 1980). A stellar wind could also confine the relativistic gas sufficiently long to provide the steady component of the synchrotron emission (see below). If such a stellar wind has a i/r 2 radial density dependence then the freefree optical depth would be unity at 0.3 GHz for a depth corresponding to r~5 x 1015 cm. Synchrotron emission from relativistic electrons embedded in the wind in this region would exhibit a radio spectrum similar to the one observed. The time dependence of the radio emission from SS ~33 indicates that t ro co° onoo w . . 0 =y component with comparable strength which is variable on a time scale of days to weeks. There are occasional cha~ges on a time scale of hours (Seaquist et al. 1979). Ken Johnston has described in this conference extensive data on variability. Therefore l shall not dwell further on the statistics of the variability but rather look at individual interesting examples and their