Cosmic ray spectrum in the energy range 1.0E15-1.0E18 eV and the second knee according to the small Cherenkov setup at the Yakutsk EAS array

TL;DR

This study analyzes cosmic ray spectra between 10^15 and 10^18 eV using Cherenkov light data from Yakutsk, identifying two spectral features called knees and discussing their implications for cosmic ray origins and composition.

Contribution

It provides a detailed measurement of the cosmic ray spectrum and composition around the second knee, highlighting differences from the first knee and suggesting cosmic ray influx from the Meta-galaxy.

Findings

Identification of the first and second knees at ~3×10^15 eV and ~10^17 eV.

Observation of a change in mass composition from lnA~3 to lnA~1.5.

Quantification of spectral index differences across the knees.

Abstract

From the data on Cherenkov light from EASs collected over the period of more than 15 years, the spectrum of cosmic rays was obtained in the energy range 1.0E15-1.0E18 eV. This spectrum has two features: at ~3.0E15 eV (first knee) and at ~1.0E17 eV (second knee). The first knee is characterized by the indexes \gamma_1=2.7+/-0.03 and \gamma_2 = 3.12+/-0.03 and the second knee - by \gamma_1 = 2.92+/-0.03 and \gamma_2=3.24+/-0.04. In the second case the difference amounts to \Delta\gamma_23=0.32{\pm}0.03{\pm}0.05 which is less than the case of the first knee \Delta\gamma_12=0.42+/-0.03+/-0.05 (here dual errors represent statistics and systematics correspondingly). The lesser difference \Delta\gamma_23 can be explained with the influx of cosmic rays from Meta-galaxy and, hence, with some increase of the cosmic rays intensity in the energy range 5.0E16-1.0E18 eV, which compensates the escape of heavier nuclei from the Galaxy. The presence of the second knee could be confirmed by abrupt change in mass composition from from lnA~3 at ~1.0E17 eV to lnA~1.5 at 1.0E18eV found from the analysis of longitudinal development of EAS.