**Publication date:** Jun 2018

**Abstract:**

Radiation reaction acting on a charged particle moving at a stable circular orbit of a magnetized black hole (BH) can lead to the shift of the orbital radius outward from the BH. The effect causes an increase of the energy and angular momentum of the particle measured by an observer at rest at infinity. In this paper, we show that „widening“ of such orbits is independent of the field configuration, but it appears only in the cases with the external Lorentz force acting outward from the BH. This condition corresponds to qLB > 0, where q and L are the charge and angular momentum of the particle, and B is intensity of the external magnetic field. As examples of the orbital widening, we consider two scenarios with an external homogeneous magnetic field and a magnetic dipole field generated by a current loop around a Schwarzschild BH. We show that the orbital widening is accompanied by quasi-harmonic oscillations of the particle, which are considerably large in the magnetic dipole fields. We also estimate the timescales of orbital widening, from which it follows that the effect can be relevant in the vicinity of stellar-mass BHs.

**Authors:**

Tursunov, A. A.; Kološ, M.; Stuchlík, Z.;