Contour plot of the radial emissivity parameter q and the spin a. The data were created by a lamp-post model with the height h = 1.5 r_g, radially stratified ionisation, and isotropic angular emissivity. A single-ionisation model with limb darkening and power-law description for the radial emissivity was used to fit the data. The best-fit parameters are indicated by a small cross within the contours.