Title:

Optical polarisation theory in the interpretation of SS433

Since the discovery of the two systems of moving emission lines in the optical spectrum of SS433, this peculiar object has stimulated intense study and speculation. A generally accepted dynamical model for the binary system comprises an earlytype supergiant and a compact object surrounded by a precessing thick accretion disk along the axis of which are ejected two opposing,mildly relativistic, collimated jets. The polarisation of the system, providing information into the distribution of matter in the system is, therefore, a valuable test for this model. Hence, in this thesis, we consider the theory of optically thin, Thomson scattering polarisation and model the observed polarisation of SS433. in order to provide further independent constraints on the dynamical model. The introductory section of this thesis is contained in the first two chapters. Chapter 1 is a review of the observations that have been made of SS433 as well as a description of the basic kinematic model and its development to the present dynamical model. In Chapter 2, we review the constraints on the system imposed by the observations, and the physi problems which arise from the observations and dynamical model. In Chapter 3, we develop the theory of Thomson electron scattering polarisation in the optically thin limit. The simple case of a point light source is extended to allow for arbitrary source shapes, the effect of which can be expressed as a 'depolarisation factor'. This factor is found explicitly in the two relevant special cases viz. a spherical (stellar) source and a luminous (accretion) disk source. The equations for the normalised Stokes Parameters for a general coro tating scattering region in a binary system are then derived. A simple geometrical model for the polarisation of SS433 is developed in Chapter 4, including the effect of spherical light sources. Although this model has many limitations, they are either not important to the polarisation or can be compensated for by smoothing the data, particularly if only the precession variation is studied and the size and orbital phasing of the data set permit those data points taken during eclipses to be omitted. Finally, we predict the broad characteristics of the polarisation, such as the expected dominant frequency in the data, based on this model. In Chapter 5, we analyse the presently available polarisation data of SS433 which span 3 1/2 years of observations. The power spectra of both the observed Q and U Stokes Parameters clearly show the precession period and its first harmonic, as expected from the previous chapter, but no other periods show above the noise. We, therefore, develop an optimisation procedure to fit a simple, precession only models to the data and compare the results of this procedure with others. Finally, we attempt to fit a model including the binary/orbital effects to the data using the bestfit parameter values of the precession only model, thereby reducing the number of free parameters. A significant reduction in the x2 parameter is found if only the first harmonic of the orbital period is added to the precession model. No reduction in x2 is found when the full model developed in Chapter 4 is fitted to the data. Chapter 6 investigates the dust grain model developed by Ramaty etal., (1984), to explain the narrow gammaray lines claimed to be observed from SS433. Although this model could, in principle, produce narrow gammaray lines, as it is presented by Ramaty et al., it is neither selfconsistent nor consistent with the observations at other wavelengths. In particular, the process which produces the gammarays is highly inefficient and the grains must, therefore radiate the waste energy deposited in them. This implies an IR flux from SS433 many orders of magnitude greater than observed. Moreover, the restrictions on the dimensions of the emitting region and the small observed polarisation imply that the emitting region must be optically thick in grains and, therefore, that the temperature of the grains would, in fact, be too high for them to survive. Chapter 7 concludes the thesis with a reprise of the conclusions and suggestions for future work arising from the detailed investigations presented here.
