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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 gamma-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, alpha ~ 2, and produce detectable gamma-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 gamma-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.

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