  High-energy astrophysical neutrino fluxes are, for many applications, modeled
as simple power laws as a function of energy. While this is reasonable in the
case of neutrino production in hadronuclear $pp$ sources, it typically does not
capture the behavior in photohadronic $p\gamma$ sources: in that case, the
neutrino spectrum depends on the properties of the target photons the cosmic
rays collide with and on possible magnetic-field effects on the secondary pions
and muons. We show that the neutrino production from known photohadronic
sources can be reproduced by a thermal (black-body) target-photon spectrum if
one suitably adjusts the temperature, thanks to multi-pion production
processes. This allows discussing neutrino production from most known $p\gamma$
sources, such as gamma-ray bursts, active galactic nuclei and tidal disruption
events, in terms of a few parameters. We apply this thermal model to study the
sensitivity of different classes of neutrino telescopes to photohadronic
sources: we classify the model parameter space according to which experiment is
most suitable for detection of a specific source class and demonstrate that
different experiment classes, such as dense arrays, conventional neutrino
telescopes, or radio-detection experiments, cover different parts of the
parameter space. Since the model can also reproduce the flavor and
neutrino-antineutrino composition, we study the impact on the track-to-shower
ratio and the Glashow resonance.
