Spinning bodies and the Poynting–Robertson effect in the Schwarzschild spacetime

Abstract

A spinning particle in the Schwarzschild spacetime deviates from geodesic behavior because of its spin. A spinless particle also deviates from geodesic behavior when a test radiation field is superimposed on the Schwarzschild background: in fact the interaction with the radiation field, i.e. the absorption and re-emission of radiation, leads to a friction-like drag force responsible for the well-known effect which exists already in Newtonian gravity, the Poynting–Robertson effect. Here the Poynting–Robertson effect is extended to the case of spinning particles by modifying the Mathisson–Papapetrou model describing the motion of spinning test particles to account for the contribution of the radiation force. The resulting equations are numerically integrated and some typical orbits are shown in comparison with the spinless case. Furthermore, the interplay between spin and radiation forces is discussed by analyzing the deviation from circular geodesic motion on the equatorial plane when the contribution due to the radiation can also be treated as a small perturbation. Finally the estimate of the amount of radial variation from the geodesic radius is shown to be measurable in principle.