Energy spectrum and mass composition of high-energy cosmic rays

Abstract

Primary cosmic rays above energies of about 100 TeV are investigated by observations of extensive air showers (EAS) using large area ground based detector installations for registering various components of the EAS cascade development. By such indirect studies of the primary cosmic rays a steepening of the power-law spectrum at around 3-5 PeV, known as the knee, has been identified. At higher energies around 5 EeV there appears a further change of the spectral index towards a flattening of the spectrum, called the ankle. The energy region above ca 50 EeV, where a cut-off of the cosmic ray spectrum (Greisen-Zatsepin-Kuz'min (GZK) cut-off) is theoretically predicted, is of particular current interest and provides an astrophysical enigma, since obviously trans-GZK events have been observed. Any explanation of these features of the cosmic ray spectrum needs sufficiently detailed knowledge of the shape of the spectrum and of the variation of the mass composition of cosmic rays. In this paper different experimental approaches deducing mass and energy sensitive information from the EAS experiments and their results are discussed. The experiments involve measurements of secondary particle distributions at various observation levels and of muons by deep underground detectors, as well as measurements of air Cherenkov light and, in particular at higher energies, of air fluorescence light emitted during the EAS development. Recently, methods for analysing multi-dimensional EAS parameter distributions have been favoured. They take into account correlations of different EAS parameters and, in particular by non-parametric techniques, also the influence of the intrinsic fluctuation of the air shower development. This paper illustrates the application of such methods in a coherent view of recent results. The advanced analysing methods are corroborated by hybrid experimental set-ups registering a larger set of different EAS observables simultaneously in an event-by-event mode. In addition such approaches provide the possibility to test the consistency of the hadronic interaction models and Monte Carlo procedures used as reference for the analyses. The physical and astrophysical implications of the current findings in various energy regions are briefly discussed and prospects of future experiments are presented.

Pulsar Wind Nebulae and Cosmic Rays: A Bedtime Story

“…These models, coupled with the ubiquity of TeV gamma-rayemitting PWNe [33,34], suggest pulsars/PWNe as a (perhaps the only) source of Galactic cosmic rays above the knee [30,32]. This is consistent with measurements that indicate a heavier cosmic ray mass composition above the knee [35].…”

Thunderstorm observations by air-shower radio antenna arrays

“…So far, LOPES has detected more than a thousand highenergy cosmic ray events serving as input on detailed analysis of the radio signal and the understanding of the radio emission in air showers in order to pave the way for large scale applications of the technique (Haungs et al, 2009). …”

“…This radio emission gives an integrated measurement all over the shower development in the atmosphere contrary to particle detectors that only record a footprint of the shower at observation level. In contrast to the fluorescence or Cherenkov detection technique, where also the longitudinal shower development is observed by means of the EAS's electromagnetic emission, radio signals can be measured at night and daytime (Haungs et al, 2003). The radio detection technique can complement the information gained by particle detectors as well as can be operated as a stand alone detection technique.…”

“…The digital radio antenna array LOPES (Falcke et al, 2005;Haungs et al, 2009) was designed and built to perform the proof-of-principle of the EAS radio detection technique. LOPES is co-located and combined with the air-shower detector KASCADE-Grande (Antoni et al, 2003;Apel et al, 2010).…”

Simple hadronic cascade simulations

“…The muon yield at detector level is a key signal to determine the energy and composition of the primary cosmic rays [1] and that is the motivation of this study.…”