Ultra-high energy cosmic rays: 40 years retrospective of continuous observations at the Yakutsk array: Part 1. Cosmic ray spectrum in the energy range 1015–1018 eV and its interpretation

Abstract

The experimental data on the cosmic ray energy spectrum obtained from the Small Cherenkov Array in Yakutsk on the measurement of Cherenkov radiation in showers with energy 1015–1018 eV are discussed. The data were obtained by means of continuous array operation since 1994. The all particle spectrum in this energy region was found to have a complex shape and cannot be described by a simple exponential function with a single slope indicator, g. After the first kink at energy 3 · 1015 eV (knee), the spectrum becomes steeper at Δγ = 0.4 up to energy 8 · 1016 eV, the slope of the spectrum is 2.92 ± 0.03 and then again changes slope by Δγ = 0.32 ± 0.05 from about ∼2· 1017 eV. The second kink in the spectrum observed at the Yakutsk EAS array at ∼2·1017 eV, or also called second knee, is a significant result for space astrophysics of ultra-high cosmic rays. In this paper we discuss possible scenarios for spectrum formation of cosmic rays by galactic sources to energies <1017 eV, mainly supernovae remnants (SNR) and Metagalactic origins in the energy range 1017–1018 eV. Most likely, that measurement of the second knee is related with the transitional region, galactic to extragalactic origin of cosmic rays.