Point-like cosmic photon sources have been observed in the TeV range by the ground based observatories . Two mechanisms are possible for the production of these photons . The electromagnetic one is based on synchrotron radiation emitted by accelerated plasma followed by inverse Compton scattering on electrons. The hadronic one is based on the decay of 's produced in hadronic interactions of accelerated nucleons with a cosmic target (matter or photon field).
The first mechanism does not produce any neutrinos. The second one, in contrast, gives rise to neutrino production coming from the decay of charged pions produced together with the neutral ones.
Possible galactic sources are:
They are made of a compact star, such as a neutron star or a black hole, which accretes the matter of its non compact companion. Strong magnetic fields combined to plasma flows lead to a stochastic acceleration of particles. The interaction of the accelerated particles with the accreted matter or the companion itself produces mesons which eventually decay into neutrinos.
Protons inside supernova shells can be accelerated in the magnetosphere of the pulsar . The interaction of these protons with the matter of the shell gives rise to neutrino emission. The active neutrino phase lasts from 1 to 10 years after the supernova explosion.
As for extra-galactic sources:
These galaxies are the brightest objects in the Universe.
Emission of -rays up to 10GeV from Active Galactic Nuclei have been well established by EGRET ; two of them (Mrk 421 and Mrk 501) have been also observed as emitters of high energy gamma rays (above 1TeV) by ground based observatories .
By their very existence, galactic and extra-galactic cosmic rays guarantee the production of high energy cosmic neutrinos. Indeed, the primary cosmic rays can interact with:
Topological defects (cosmic strings, monopoles)  and non baryonic dark matter  bring into play very heavy entities which by quantum evaporation, collapse or annihilation eventually give rise to an emission of high energy neutrinos.