Indirect observational constraints on UHECR source environments*

We present new constraints on the environments surrounding UHECR accelerators, obtained by combining a realistic and flexible phenomenological UHECR source model with the latest UHECR spectrum & composition data and constraints from neutrinos and gammas. Our analysis provides a new set of criteria on the magnetic field, size, and temperature of UHECR source environments. These criteria (distinct from the Hillas criterion for the UHECR accelerator) will be compared to observational data to infer which astrophysical source candidates are preferred or excluded by multimessenger data.