2.1. A brief low energy perspective
Supernova remnants (SNRs) are thought to be the main source of
both CR ions and electrons with energies below the knee. The particle
acceleration mechanism
in individual SNRs is usually assumed to be diffusive shock
acceleration, which naturally leads to a power-law population of
relativistic particles. In the standard version of this mechanism
(e.g. [132]),
particles are scattered by magnetohydrodynamic
waves repeatedly through the shock front. If they encounter an
enhancement of molecular density, the pion channel can lead to
observable amounts of
-rays (see
Ref. [133]
for a review, and references therein for details). Electrons
suffer synchrotron losses, producing the non-thermal emission from
radio to X-rays usually seen in shell-type SNRs. The maximum
energy achieved depends on the shock speed and age as well as on
any competing loss processes. In young SNRs, electrons can easily
reach energies in excess of 1 TeV, where they produce X-rays by
synchrotron mechanism (see, for example,
[134,
135]).
CRs of low energies are also expected to be accelerated in OB
associations, through turbulent motions and collective effects of
star winds (e.g.
[136,
137]).
The main acceleration region
for TeV particles would be in the outer boundary of the
supperbubble produced by the core of a given stellar association.
If there is a subgroup of stars located at the acceleration
region, their winds might be illuminated by the locally
accelerated protons, which would have a distribution with a slope
close to the canonical value,
~ 2, and produce
detectable
-rays
[138].
The HEGRA detection in the vicinity of Cygnus OB2, TeV
J2032+4131
[139],
could be, judging from multiwavelength observations
[140],
the result of such a process
[138].
A nearby EGRET source (3EG J2033+4118) has also a likely stellar origin
[141,
142,
143].
Truth is, as always, bitter. No astrophysical source of UHECRs,
nor of CRs with energies below the knee, has been ever confirmed.
Out of all SNRs coinciding with non-variable
-ray
sources detected by EGRET
[144,
145,
146,
147],
the supernova remnant RX J1713.7-3946 is perhaps one the most
convincing cases for a hadronic cosmic-ray accelerator detected so
far in the Galaxy, although yet subject to confirmation
[148,
149,
150,
151,
152].
Other excellent candidates include SN1006 (e.g.
[153])
and Cas A (e.g.
[154]).
Kilometer-scale neutrino telescopes have also been proposed as viable
detectors of hadronic CR sources (e.g.
[155,
156,
157,
158]),
and will be a welcomed addition to the space- and ground-based
detectors already existing or planned.