We constrain properties of ultrahigh energy cosmic ray source environments (and potentially astrophysical neutrino sources), including their photon temperature, gas density, size, magnetic field strength and coherence length, using UHECR and neutrino spectra and composition. Our analysis represents a new type of information on UHECR sources, independent of the mechanism responsible for the UHECR acceleration. We also explore the possibility of a common origin of UHECRs and astrophysical neutrinos and further constrain sources which are consistent with this possibility. We show that the common origin hypothesis can only be satisfied for certain hadronic interaction models, showing that multimessenger analyses have the power to also constrain hadronic physics beyond LHC energies.
Ultrahigh energy cosmic rays; astrophysical neutrinos; multimessenger astrophysics; cosmic ray sources