Active Galactic Nuclei as Potential Sources of Ultra-High Energy Cosmic Rays

Abstract

Active Galactic Nuclei (AGNs) and their relativistic jets belong to the most promising class of ultra-high-energy cosmic ray (UHECR) accelerators. This compact review summarises basic experimental findings by recent instruments, and discusses possible interpretations and astrophysical constraints on source energetics. Particular attention is given to potential sites and mechanisms of UHECR acceleration in AGNs, including gap-type particle acceleration close to the black hole, as well as first-order Fermi acceleration at trans-relativistic shocks and stochastic shear particle acceleration in large-scale jets. It is argued that the last two represent the most promising mechanisms given our current understanding, and that nearby FR I type radio galaxies provide a suitable environment for UHECR acceleration.

How, where and when do cosmic rays reach ultrahigh energies?

“…The power requirement is therefore sometimes referred to as a 'Hillas-Lovelace limit' (although see also Refs. [45][46][47][48][49]). The basic idea is that a source must be able to supply enough magnetic energy per unit time that a given product 𝛽𝐵𝑅 can be maintained in the acceleration site.…”

“…However, such a boost may potentially be important particularly if GRB internal shock or afterglow models are to reach rigidities of 10 EV (see section 4.2.3). The power requirement can also include an additional Γ 2 factor [49,52], although this cancels with the outflow opening angle, Θ, if Θ ∝ Γ −1 . In any case, it is hard for a relativistic accelerator to reach optimal conditions with 𝜂 ≈ 1 (see section 3.2.1), and so we take equations 3 and 4 as our basic energetic requirements.…”

“…Shear acceleration involves scattering across a shear layer [75], in a similar manner to shock acceleration, and has been proposed as a possible mechanism for UHECR acceleration at the edge of a relativistic jet. A detailed discussion of shear acceleration pertaining to UHECR acceleration in AGN jets is given by Rieger in a recent review [49], who highlights some recent studies proposing one-shot or 'espresso' acceleration in relativistic AGN jets [76][77][78]. Alternatively, shear acceleration can be rather gradual, and the details of the process depend on the thickness of the shear layer and the Kelvin-Helmholtz instabilities operating in the region [49,79,80].…”

“…A detailed discussion of shear acceleration pertaining to UHECR acceleration in AGN jets is given by Rieger in a recent review [49], who highlights some recent studies proposing one-shot or 'espresso' acceleration in relativistic AGN jets [76][77][78]. Alternatively, shear acceleration can be rather gradual, and the details of the process depend on the thickness of the shear layer and the Kelvin-Helmholtz instabilities operating in the region [49,79,80]. Various authors have also discussed second-order Fermi acceleration (Fermi II) by MHD turbulence in, for example, giant radio lobes [81-83, see also section 4.2.2].…”

How, where and when do cosmic rays reach ultrahigh energies?

“…Shear acceleration involves scattering across a shear layer [75], in a similar manner to shock acceleration, and has been proposed as a possible mechanism for UHECR acceleration at the edge of a relativistic jet. A detailed discussion of shear acceleration pertaining to UHECR acceleration in AGN jets is given by Rieger in a recent review [49], who highlights some recent studies proposing one-shot or 'espresso' acceleration in relativistic AGN jets [76][77][78]. Alternatively, shear acceleration can be rather gradual, and the details of the process…”