  The diffuse neutrino flux measured in IceCube is comparable with the
ultrahigh-energy cosmic ray (UHECR) flux, which has led to the concept of a
unified origin of high-energy neutrino and UHECR backgrounds. We construct a
generic unification model of sources to explain UHECR data at $\gtrsim 10^{19}$
eV, and high-energy neutrinos with energies that exceed $\sim100$ TeV in the
framework of photo-meson production processes, and provide general constraints
on the source properties. A source environment with moderately efficient
in-situ production of $\gtrsim 100$ TeV neutrinos with an optical depth of $0.1
\lesssim \tau_{p\gamma}\lesssim 0.6$ must be realized to accelerate cosmic rays
to ultrahigh energies. The measured fluxes of cosmic rays and neutrinos set a
bound on the source luminosity and its rate density. Although the results are
rather general and applicable to unknown source population, among the proposed
source candidates, low-luminosity gamma-ray bursts (GRBs) and tidal disruption
events (TDEs) could satisfy the requirements if the Lorentz bulk factor of
plasma outflow and the equipartition parameters for cosmic rays and magnetic
field are appropriately selected.
