Proc. Neutrino Astrophysics - MPP Theory Group

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in gamma-rays. However, the acceleration of particles to high energies generally requires a
low-density plasma. The acceleration of cosmic rays above ∼ 10 18.5 TeV in sources belonging
to the Milky Way seems impossible. Among the prime candidates for these ultra-high energy
cosmic rays are (radio and gamma-ray emitting) active galactic nuclei [5] and gamma ray
bursts [6]. In those sources cooling of accelerated protons (ions dissociate over cosmological
distances) is dominated by interactions with low-energy synchrotron photons (from the accelerated
electrons). Due to electromagnetic cascading most gamma-rays are shifted to below
the TeV range whereas the neutrinos remain unaffected from such reprocessing. Hence the
γ/ν-ratio decreases with energy in spite of being a constant function (from decay kinematics)
at the production site (if proton-matter collisions dominate the energy loss, the γ/ν-ratio in
the TeV range may be constant and of order unity [7]. The total electromagnetic power of the
gamma-ray emitting active galactic nuclei can explain the observed extragalactic gamma-ray
background and is of the same order as the power in an extragalactic E −2 differential cosmic
ray spectrum dominating above 10 6.5 TeV. Therefore, neutrino flux predictions are generally
bounded by ∼ 10 −6 GeV cm −2 s −1 st −1 (Fig. 1). The models [8, 9] yield ∼ 300 upward events
above TeV per year per km 3 .
Figure 1: Overview of theoretical neutrino flux predictions as compiled by R.J. Protheroe
during a workshop on high-energy neutrino astrophysics in Aspen, 1996. Labels refer to (a)
atmospheric neutrinos (the solid part indicates the part of the spectrum measured by the
Frejus experiment), (b) galactic disk in center and anti-center directions (due to Domokos),
(c) AGN [11], (d) AGN [8], (e) AGN [9], (f) charm production upper limit [12], (g) GRBs
(due to Lee), (h) UHE CRs (due to Stecker), (i) TDs [10] (courtesy of W. Rhode).