TL;DR
This paper reviews how active galactic nuclei and their jets could accelerate ultra-high-energy cosmic rays, emphasizing the importance of specific acceleration mechanisms and the need for better observational data.
Contribution
It discusses the relevance of various acceleration sites and mechanisms in AGN jets, highlighting Fermi-type acceleration as the most promising for UHECR production.
Findings
Fermi-type acceleration at shocks and shear flows is promising for UHECRs.
Current data on UHECR sources and directions are insufficient for definitive conclusions.
AGN jets are plausible sources of ultra-high-energy cosmic rays.
Abstract
Active Galactic Nuclei (AGN) and their relativistic jets are believed to be potential sites of ultra-high-energy (UHE) cosmic ray acceleration. This paper reviews basic observational findings as well as requirements on source energetics, and then discusses the relevance of different acceleration sites and mechanisms, such as black hole gap, shock in back-flows or jet shear acceleration. When put in context, the result suggests that Fermi-type particle acceleration at trans-relativistic shocks and/or in shearing, relativistic flows offers the most promising framework for UHECR production in AGN. Truly deciphering the astrophysical sources of UHECRs, however, still needs improved statistical information on arrival directions and source correlations.
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