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