Non-monotonic enhancement of the Magnetic Penrose Process in Kerr-Bertotti-Robinson spacetime and its implication for electron acceleration

Abstract We studied the magnetic Penrose process (MPP) in the Kerr-Bertotti-Robinson (KBR) spacetime, an exact rotating electrovacuum solution describing a black hole (BH) immersed in an intrinsic, uniform electromagnetic field. We analyze the behavior of charged particles in this geometry and find that the spacetime structure itself responds non-monotonically to the background magnetic field B . Specifically, both the event horizon and the static limit surface first expand as B increases, reach a maximum size at an intermediate field strength, and then contract toward the extremal limit. Although the ergoregion itself shrinks monotonically with B , this structural feature gives rise to a pronounced non-monotonic dependence of the energy extraction efficiency on the magnetic field B , i.e., the efficiency initially rises, attains a maximum value, and subsequently falls as the extremal condition is approached. This contrasts sharply with the monotonic trends usually associated with magnetic enhancements in the Kerr geometry. We further explore an astrophysical application of the MPP by estimating the maximum energy of electrons escaping from the ergoregion of the KBR BH. Modeling neutron beta decay occurring near the event horizon, we derive an analytical expression for the energy gained by electrons accelerated by the magnetic field. Applying our results to the supermassive BH at the Galactic center, SgrA^*, we find that electrons can be accelerated up to energies of ∼ 10 15 eV for realistic values of the spin and magnetic field. Although these energies exceed the observed upper range of cosmic-ray electrons, radiative losses such as synchrotron emission and inverse-Compton scattering can efficiently reduce them to the observed TeV scale. The KBR BH thus provides a clear example of how intrinsically electromagnetic fields can qualitatively affect the BH energetics, introducing an optimal magnetic field strength for rotational energy extraction, a feature with potential implications for high-energy astrophysical environments where magnetic fields are present. Our results show that in electrovacuum spacetimes where the magnetic field is intrinsic to the geometry, the MPP can be significantly modified, revealing a new regime of BH energy extraction.

Перевод пока недоступен