Variation of azimuthal component of four velocity, dφ/ dτ, with radial distance. From top to bottom: solid black curve corresponds to dφ/ dτ for the test particle in a sequence of circular orbits at different radii in the range, thick dashed (blue) curve, dφ/ dτ for a test particle following spiral trajectory for initial luminosity k₁ = 0.36; red dashed dotted curve for k₁ = 0.48; and magenta curve for k₁ = 0.60.