Phase topology of one irreducible integrable problem in the dynamics of a rigid body

We consider the integrable system with three degrees of freedom for which V. V. Sokolov and A. V. Tsiganov specified the Lax pair. The Lax representation generalizes the $L$–$A$ pair found by A. G. Reyman and M. A. Semenov-Tian-Shansky for the Kovalevskaya gyrostat in a double field. We give explicit formulas for the additional first integrals $K$ and $G$ (independent almost everywhere), which are functionally related to the coefficients of the spectral curve for the Sokolov–Tsiganov $L$–$A$ pair. Using this form of the additional integrals $K$ and $G$ and the Kharlamov parametric reduction, we analytically present two invariant four-dimensional submanifolds where the induced dynamical system is Hamiltonian (almost everywhere) with two degrees of freedom. The system of equations specifying one of the invariant submanifolds is a generalization of the invariant relations for the integrable Bogoyavlensky case (rotation of a magnetized rigid body in homogeneous gravitational and magnetic fields). We use the method of critical subsystems to describe the phase topology of the whole system. For each subsystem, we construct the bifurcation diagrams and specify the bifurcations of the Liouville tori both inside the subsystems and in the whole system.