The long-term rotation dynamics of neutron stars with differentially rotating unmagnetized core

We consider the pulsar long-term rotation dynamics taking into account the non-rigidity of neutron star rotation. We restrict our attention to the models with two essential assumptions: (1) crust-core interaction occurs via the viscosity (magnetic coupling is not important); (2) neutron star shape is symmetrical over the magnetic axis. The neutron star core is described by linearized quasi-stationary Newtonian hydrodynamical equations in one-fluid and two-fluid (neutron superfluidity) approximations. It is shown that in this case the pulsar inclination angle evolves to 0 degrees or 90 degrees very quickly. Since such fast evolution seems to contradict the observation data, either neutron stars are triaxial or the magnetic field plays the leading role in crust-core coupling.