Centrifugal acceleration of protons by a supermassive black hole

Abstract

ABSTRACT Centrifugal acceleration is due to the rotating poloidal magnetic field in the magnetosphere that creates the electric field which is orthogonal to the magnetic field. Charged particles with finite cyclotron radii can move along the electric field and receive energy. Centrifugal acceleration pushes particles to the periphery, where their azimuthal velocity reaches the speed of light. We calculated particle trajectories by numerical and analytical methods. The maximum obtained energies depend on the parameter of the particle magnetization κ, which is the ratio of rotation frequency of magnetic field lines in the magnetosphere ΩF to non-relativistic cyclotron frequency of particles ωc, κ = ΩF/ωc <<1, and on the parameter α which is the ratio of toroidal magnetic field BT to the poloidal one BP, α = BT/BP. It is shown that for small toroidal fields, α < κ1/4, the maximum Lorentz factor γm is only the square root of magnetization, γm = κ−1/2, while for large toroidal fields, α > κ1/4, the energy increases significantly, γm = κ−2/3. However, the maximum possible acceleration, γm = κ−1, is not achieved in the magnetosphere. For a number of active galactic nuclei, such as M87, maximum values of Lorentz factor for accelerated protons are found. Also, for special case of Sgr. A*, estimations of the maximum proton energy and its energy flux are obtained. They are in agreement with experimental data obtained by HESS Cherenkov telescope.