Proc. Neutrino Astrophysics - MPP Theory Group

Astrophysical Sources of High-Energy Neutrinos
K. Mannheim
Universitäts-Sternwarte, Geismarlandstraße 11,
D-37083 Göttingen, Germany (kmannhe@uni-sw.gwdg.de)
This contribution reviews currently discussed astrophysical sources of high-energy (>TeV)
neutrinos. Particle astrophysics with high-energy neutrinos allows to study electroweak interactions,
neutrino properties, particle acceleration theories, cosmology, dark matter candidates,
and the enigmatic sources of cosmic rays [1]. The detection of high-energy neutrinos
is greatly facilitated by the rising neutrino cross section and muon range which cause the
detection probablity for neutrinos to increase with energy. From an astrophysical point of
view, the existence of sources of high-energy neutrinos seems very likely, if not guaranteed,
for two strong reasons.
First of all, there are numerous cosmic synchrotron sources (radio, optical, X-ray) for
which simple arguments give electron energies in the GeV–TeV range [2]. Since most electromagnetic
particle acceleration mechanisms predict accelerated baryons along with the accelerated
electrons, hadronic energy losses due to pion production will lead to high-energy
neutrino emission. If the maximum energies of particles emerging from cosmic accelerators
are determined by the balance between energy gains and energy losses, baryons must reach
much higher energies than electrons since their energy losses are much weaker than those of
the electrons at the same energy. Therefore, pion energies may be expected to reach energies
even above TeV. Neutrinos from pion decay will, of course, also be very energetic, since
each flavor carries ∼ 1/4th of the decaying pion energy. Baryon acceleration could only be
avoided in a plasma exclusively composed of electrons and positrons. Although pair plasmas
probably exist in pulsars, gamma-ray bursts, and active galactic nuclei, baryon pollution
(from the neutron star surface or due to entrainment of environmental plasma) changes the
picture quite radically: As the pair plasma expands most of the internal energy is quickly
converted into kinetic energy of the polluting baryons which then has to be tapped in a
second stage dissipative process to give rise to the observed radiation. It has been widely
discussed in the context of pair fireball models for gamma-ray bursts [3] that the observed
non-thermal gamma-ray spectra generally require emission from a baryonic bulk flow at a
large distance to the energizing compact object rather than from a pair plasma close to the
compact object itself.
Secondly, we know baryonic cosmic rays exist (up to energies of 10 8 TeV) and they must
come from somewhere. Supernova remnants are still among the prime candidates for the acceleration
of the cosmic rays below ∼ 10 3 TeV, they should therefore be visible as gamma-ray
and neutrino sources at comparable flux levels. This flux level is beyond reach for the first
generation of high-energy neutrino experiments. Giant molecular clouds could be somewhat
more important, since they provide most of the target mass in the vicinity of star forming
regions where supernovae occur. Cosmic rays propagating through the Milky Way produce
pions in hadronic interactions with the interstellar material and can be traced in gamma-rays.
A diffuse flux of neutrinos at a flux comparable to the gamma-ray flux should be seen from the
Galactic plane [4] corresponding to ∼ 150 upward events above one TeV per year in a detector
with an effective volume of 1 km 3 (compared with 6.5 ×10 3 atmospheric background events).
One can also speculate about enshrouded sources which only show up in neutrinos but not
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