  There are tantalizing hints that jets, powered by supermassive black holes at
the center of active galaxies, are true cosmic proton accelerators. They
produce photons of TeV energy, possible higher, and may be the enigmatic source
of the highest energy cosmic rays. Photoproduction of neutral pions by
accelerated protons on UV light is the source of the highest energy photons, in
which most of the bolometric luminosity of the galaxy may be emitted. The case
that proton beams power active galaxies is, however, far from conclusive.
Neutrinos from the decay of charged pions represent an uncontrovertible
signature for the proton induced cascades. We show that their flux can be
estimated by model-independent methods, based on dimensional analysis and
textbook particle physics. Our calculations also demonstrate why different
models for the proton blazar yield very similar results for the neutrino flux,
consistent with the ones obtained here.
