SIMBAD references

We investigate the X-ray colour-colour variations of Z sources. We use a Monte Carlo code to model the scattering of radiation emitted from the hot central corona (HCC) as it passes through a (non-uniform) radially accreting region. We find that the horizontal branch of the Z track of Cyg X-2 can be produced by changes in the radial accretion structure with increasing mass transfer rate. The normal brancht however, requires changes in the spectrum emerging from the HCC. The effect of the non-uniform bulk motion of the plasma is shown to be important in determining both the position of the Z track in colour-colour space, and also the extent and orientation of the horizontal branch.