Centrifugal acceleration of protons by a supermassive black hole

The centrifugal acceleration is due to the rotating poloidal magnetic field in the magnetosphere 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 light speed. We have 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 $Ω_F$ to non-relativistic cyclotron frequency of particles $ω_c$, $κ= Ω_F /ω_c << 1$, and from the parameter $α$ which is the ratio of toroidal magnetic field $B_T$ to the poloidal one $B_P$, $α= B_T / B_P$. It is shown that for small toroidal fields, $α<κ^{1/4}$, the maximum Lorentz factor $γ_m$ is only the square root of magnetization, $γ_m = κ^{-1/2}$, while for large toroidal fields, $α>κ^{1/4}$, the energy increases significantly, $γ_m = κ^{-2/3}$. However, the maximum possible acceleration, $γ_m = κ^{-1}$, 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.

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