Particle acceleration in the vacuum gaps in black hole magnetospheres

¹ ISDC, Astronomy Department, University of Geneva, Ch. d’Ecogia 16, 1290 Versoix, Switzerland
e-mail: ptitsyna.k@gmail.com
² Institute for Nuclear Research of the Russian Academy of Sciences, 60th October Anniversary Prospect 7a, 117312 Moscow, Russia
³ Physics Department, M.V. Lomonosov Moscow State University, 119991 Moscow, Russia

Received: 13 October 2015
Accepted: 2 February 2016

Abstract

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.