  Sources of high-energy cosmic rays are presently unknown, but can be
constrained in various ways. Some of these constraints can be graphically
presented on the so-called Hillas diagram. Previous versions of this diagram
determined the range of geometrical sizes and magnetic fields of potential
astrophysical accelerators, taking into account geometrical criteria and
radiation losses. In this work, we update the Hillas diagram for protons,
taking into account the losses associated with the $p\gamma$ and photopair
interactions, relating the allowed regions to the source electromagnetic
luminosity. The strongest constraints are obtained for bright compact sources
such as the central regions of active galactic nuclei.
