Ultra-high energy cosmic ray accelerators: Population diversity and the role of ultra-fast outflows in active galactic nuclei

We don’t yet know the sources of ultra-high-energy cosmic rays (UHECRs), but an even more basic question is whether there are many sources or just a few and whether they are alike or exhibit significant differences. In this talk, I will discuss the level of diversity in the UHECR source population consistent with the latest experimental datasets and show that the sources of UHECRs are required to be surprisingly similar. I will then explore a new potential site of UHECR acceleration in non-jetted active galactic nuclei (AGN) that satisfies these population-diversity constraints: the wide-angle relativistic winds known as ultra-fast outflows. Additional support for the idea that these environments are efficient cosmic-ray accelerators comes from the study of several recently discovered Galactic Pevatrons, which, despite being smaller and less extreme, form wind-bubble structures similar to those expected in AGN ultra-fast outflows.