摘要：Aims. We consider particle acceleration in the vacuum gaps in
magnetospheres of black holes powered by the Blandford-Znajek mechanism and embedded in
the radiatively-inefficient accretion flow (RIAF) environment. In this situation, the gap
height is limited by the onset of gamma-gamma pair production on the infrared photons
originating in the RIAF.
Methods. We numerically calculated the acceleration and propagation of
charged particles by taking the detailed structure of the electric and magnetic fields in
the gap and in the entire black hole magnetosphere into account, as well as the radiative
energy losses and interactions of γ-rays produced by the propagated charged particles
with the background radiation field of the RIAF.
Results. We show that the presence of the vacuum gap has clear
observational signatures. The spectra of emission from gaps embedded in a relatively
high-luminosity RIAF are dominated by the inverse Compton emission with a sharp,
super-exponential cut-off in the very-high-energy gamma-ray band. The cut-off energy is
determined by the properties of the RIAF and is largely independent of the structure of
magnetosphere and geometry of the gap. The spectra of the gap residing in low-luminosity
RIAFs are dominated by synchrotron or curvature emission with the spectra extending into
1−100 GeV energy range. We
also consider the effect of possible acceleration of protons in the gap and find that
proton energies could reach the ultra-high-energy cosmic ray (UHECR) range only in
extremely low-luminosity RIAFs.
关键词：galaxies: active;gamma rays: galaxies;acceleration of particles;black hole physics
