Physical conditions in potential sources of ultra-high-energy cosmic   rays: Updated Hillas plot and radiation-loss constraints
  We review basic constraints on the acceleration of ultra-high-energy (UHE) cosmic rays (CRs) in astrophysical sources, namely the geometrical (Hillas) criterion and restrictions from radiation losses in different acceleration regimes. Using the latest available astrophysical data, we redraw the Hillas plot and figure out potential UHECR accelerators. For the acceleration in central engines of active galactic nuclei, we constrain the maximal UHECR energy for a given black-hole mass. Among active galaxies, only the most powerful ones, radio galaxies and blazars, are able to accelerate protons to UHE, though acceleration of heavier nuclei is possible in much more abundant lower-power Seyfert galaxies. 
Ksenia Ptitsyna and Sergey Troitsky