Nonthermal Processes in Black Hole-Jet Magnetospheres — Invited Review

Abstract

The environs of supermassive black holes are among the universe's most extreme phenomena. Understanding the physical processes occurring in the vicinity of black holes may provide the key to answer a number of fundamental astrophysical questions including the detectability of strong gravity effects, the formation and propagation of relativistic jets, the origin of the highest energy gamma-rays and cosmic rays, and the nature and evolution of the central engine in active galactic nuclei (AGN). As a step towards this direction, this paper reviews some of the progress achieved in the field based on observations in the very high energy domain. It particularly focuses on nonthermal particle acceleration and emission processes that may occur in the rotating magnetospheres originating from accreting, supermassive black hole systems. Topics covered include direct electric field acceleration in the black hole's magnetosphere, ultra-high energy cosmic ray production, Blandford-Znajek mechanism, centrifugal acceleration and magnetic reconnection, along with the relevant efficiency constraints imposed by interactions with matter, radiation and fields. By way of application, a detailed discussion of well-known sources (Sgr A*; Cen A; M87; NGC1399) is presented.

Scenarios for Ultrafast Gamma-Ray Variability in AGN

“…To illustrate the physical reason for theexistence of this factor, Broderick & Tchekhovskoy (2015) computed the divergence of the electric field in the gap. However, as the gap electric field they adopted afield determined by an expression similar to Equation (6), i.e., a value that overestimates the true field by the factor R/h (see, e.g., Blandford & Znajek 1977;Levinson & Rieger 2011;Rieger 2011, for a more accurate introduction of the electric field in the gap). Thus, the factor suggested by Broderick & Tchekhovskoy (2015) seems to be strongly overestimated.…”

“…Finally, it has been suggested that the flares can be produced in the BH magnetosphere (Neronov & Aharonian 2007;Levinson & Rieger 2011;Rieger 2011). In this case, the production site does not move relativistically withrespect to the observer, and Equation (2) is reduced to t l > t var 0 , where l = R r g .…”

Hadronic origin of the TeV flare of M87 in April 2010

“…This scenario can explain the 2010 flare but does not provide a quantitative prediction of the light curve of the flare. Similarly, the magnetosphere model [21][22][23] can explain the hard TeV spectrum but in this case also there is no detailed quantitative predication for the VHE light curve. The work by Cui et al [24] explains the VHE gamma ray flare in an external inverse Compton model with a very wide jet.…”

Gamma-rays from non-blazar AGN

“…the hadronic interaction of relativistic protons with ambient matter in the large-scale (halo-size) region ("Fermi-bubble like") [32], the integrated gammaray contribution from a supposed population of millisecond pulsars [33], or (leptonic) inverse-Compton processes with various photon fields (SSC, starlight, CMB, EBL) in the kpc-scale jet of Cen A [34,35], (iii) inner (pc-scale an below) jet models such a multiple (leptonic) SSC-emitting components moving at different angles to the line of sight [36], photo-meson (pγ) interactions of ultra-high energy protons in strong photons fields (e.g. standard disk-type) on inner jet scales [37,38,39] and elaborated lepto-hadronic variants thereof [40,41], or γ-ray induced pair-cascades in a putative dusty torus-like region [42], last not least (iv) magnetospheric scenarios based on leptonic inverse Compton processes in an under-luminous ADAF-type environment [43,44]. All of these models come with some challenges, from e.g.…”