Progress towards characterizing ultrahigh energy cosmic ray sources
  We use a multimessenger approach to constrain realistic mixed composition
models of ultrahigh energy cosmic ray sources using the latest cosmic ray,
neutrino, and gamma-ray data. We build on the successful
Unger-Farrar-Anchordoqui 2015 (UFA15) model which explains the shape of the
spectrum and its complex composition evolution via photodisintegration of
accelerated nuclei in the photon field surrounding the source. We explore the
constraints which can currently be placed on the redshift evolution of sources
and the temperature of the photon field surrounding the sources. We show that a
good fit is obtained to all data either with a source which accelerates a
narrow range of nuclear masses or a Milky Way-like mix of nuclear compositions,
but in the latter case the nearest source should be 30-50 Mpc away from the
Milky Way in order to fit observations from the Pierre Auger Observatory. We
also ask whether the data allow for a subdominant purely protonic component at
UHE in addition to the primary UFA15 mixed composition component. We find that
such a two-component model can significantly improve the fit to cosmic ray data
while being compatible with current multimessenger data.
