Flywheels; rapidly spinning, magnetized neutron stars in spherical accretion

The behaviour of a rapidly spinning, magnetized neutron star embedded in plasma accreting spherically at a supercritical rate is investigated. Friction between the magnetosphere and the surrounding plasma is a source of power. In addition, when the magnetic field is asymmetric with respect to the rotation axis of the star, the rotating magnetosphere acts as a propeller, transferring kinetic energy to the material outside the magnetosphere. We quantify these processes and calculate the values of the Alfven radius and of the density of the surrounding plasma. We find that, if the neutron star rotates rapidly and has a strong magnetic field, such objects manifest themselves as non-thermal X-ray sources. Otherwise the material outside the magnetosphere shrouds them in such high optical depth that any outgoing radiation is thermalized. (author)