Gap-type Particle Acceleration in the Magnetospheres of Rotating Supermassive Black Holes

The detection of rapidly variable gamma-ray emission in active galactic nuclei has generated renewed interest in magnetospheric particle acceleration and emission scenarios. In order to explore its potential, we study the possibility of steady gap acceleration around the null surface of a rotating black hole magnetosphere. We employ a simplified (1D) description along with the general relativistic expression of Gauss's law, and we assume that the gap is embedded in the radiation field of a radiatively inefficient accretion flow. The model is used to derive expressions for the radial distribution of the parallel electric field component, the electron and positron charge density, the particle Lorentz factor, and the number density of $\gamma$-ray photons. We integrate the set of equations numerically, imposing suitable boundary conditions. The results show that the existence of a steady gap solution for a relative high value of the global current is in principle possible if charge injection of both species is allowed at the boundaries. We present gap solutions for different choices of the global current and the accretion rate. When put in context, our results suggest that the variable very high energy $\gamma$-ray emission in M87 could be compatible with a magnetospheric origin.Comment: 20 pages, 11 figures; ApJ accepted; minor typos fixed to match published versio

Gamma-ray emission from the black hole's vicinity in AGN

Non-thermal magnetospheric processes in the vicinity of supermassive black holes have attracted particular attention in recent times. Gap-type particle acceleration accompanied by curvature and Inverse Compton radiation could in principle lead to variable gamma-ray emission that may be detectable with current instruments. We shortly comment on the occurrence of magnetospheric gaps and the realisation of different potentials. The detection of rapid variability becomes most instructive by imposing a constraint on possible gap sizes, thereby limiting extractable gap powers and allowing to assess the plausibility of a magnetospheric origin. The relevance of this is discussed for the radio galaxies Cen A, M87 and IC310. The detection of magnetospheric gamma-ray emission generally allows for a sensitive probe of the near-black-hole region and is thus of prime interest for advancing our understanding of the (astro)physics of extreme environmentsComment: Talk presented at the 7th Fermi Symposium, Garmisch-Partenkirchen, October 201

Nonthermal Processes Near Supermassive Black Holes

In recent years, γ-ray astronomy has made considerable progress in the exploration of the extragalactic γ-ray sky. In particular, active galaxies, whose relativistic jets/outflows are significantly inclined with respect to the lineof- sight, have revealed remarkable flaring activity at γ-ray energies. The observed rapid variability of the γ-ray emission, comparable to timescales of the light travel time across the black hole horizon, provides a strong motivation for testing radiative scenarios associated with the vicinity of the central supermassive black hole. In this doctoral study, we explore the so-called black hole magnetospheric scenario. Accordingly, strong particle acceleration may occur within the black hole magnetosphere in regions of unscreened electric fields (gaps). This can happen either at the null surface across which the charge density changes sign or at the stagnation surface which separates the inwardly from the outwardly moving matter. The acceleration of leptons is accompanied by γ-ray emission via inverse Compton scattering of the ambient (disk) soft photons as well as curvature radiation. This thesis explores the potential of these processes to account for the observed γ-ray features. By developing and studying an one-dimensional, steady model for magnetospheric particle acceleration and emission, as well as, estimating the terminal Lorentz factors of the accelerated charges and the maximum extractable gap power, we find that magnetospheric processes can be responsible for the observed, rapidly variable very-high-energy γ-ray emission in the radio galaxy M87