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gravitation – relativistic processes – black-hole physics – accretion discs – acceleration of particles

Introduction
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Motion of test particles under the combined effects of gravity and radiation is of obvious astrophysical significance, mainly in the case of the rarified atmosphere around a bright compact source. In the literature, such a motion has mostly been studied while approximating the particle-radiation interaction by a Thomson-like term which specifies, through an effective cross-section constant, what part of the radiation’s relative momentum is transferred to the particle. Adopting this approach, we have analyzed the “Poynting-Robertson effect" of radiation drag in the equatorial plane of the Schwarzschild and Kerr background space-times, for an outgoing or ingoing “radial" photon flux with zero or non-zero angular momentum [@BiniJS-09; @BiniGJSS-11]. Then we have also considered [@BiniGJS-11] the case of a non-test flux involved in the [*exact*]{} Vaidya solution, describing a spherically symmetric centre emitting or accreting radiation. These papers may be consulted for a wider review of literature on this topic.

In the meantime, several new contributions to the subject have appeared. [@OhKL-10] presented a numerical treatment of particle trajectories in the radiation field of a slowly rotating Kerr-like source, where the existence of equilibrium 