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The cosmic ray spectrum |
Bridging the Gap
Cherenkov telescopes can contribute to cosmic ray physics by
detecting these particles directly. CTA can provide measurements of
spectra of cosmic-ray electrons and nuclei in the energy regime where
balloon- and space-borne instruments run out of statistics.
The composition of cosmic rays up to around 100 TeV has been measured by balloon- and space-borne instruments (e.g. TRACER), and instruments such as KASCADE
can detect air showers at ground level from progenitor particles of
energy 1 PeV and higher. However, such air shower experiments have
difficulties identifying individual nuclei, and consequently their
measurements of cosmic ray composition are of lower resolution than
direct measurements.
Cherenkov telescopes are the most promising instruments to close this
experimental gap between the TeV and PeV domain, and will likely
achieve better mass resolution than ground-based particle arrays.
Cosmic Ray Electrons
In addition, CTA can perform crucial measurements of the spectrum of
cosmic-ray electrons. TeV electrons have very short lifetimes and thus
short propagation distances. The upper end of the electron spectrum
(which is not accessible by current balloon and satellite experiments)
is therefore expected to be dominated by local electron accelerators and
the cosmic-ray electron spectrum can provide valuable information about
the characteristics of the contributing sources and electron
propagation. While such measurements involve analyses that differ from
the conventional gamma-ray analysis, a proof-of-principle has already
been performed with the H.E.S.S. telescopes. The increase in sensitivity expected from CTA will provide significant improvements on such measurements.
Further Reading
Aharonian et al. (H.E.S.S. Collaboration), Probing the ATIC peak in
the cosmic-ray electron spectrum with H.E.S.S (2009), Astronomy and
Astrophysics, 508, 2, 2009, p.561-564; Available via http://arxiv.org/abs/0905.0105