Ankle-like feature in the energy spectrum of light elements of cosmic rays observed with KASCADE-Grande

Recent results of the KASCADE-Grande experiment provided evidence for a mild knee-like structure in the all-particle spectrum of cosmic rays at $E={10}^{16.92\pm 0.10}\mathrm{eV}$, which was found to be due to a steepening in the flux of heavy primary particles. The spectrum of the combined components of light and intermediate masses was found to be compatible with a single power law in the energy range from ${10}^{16.3}$ to ${10}^{18}\mathrm{eV}$. In this paper, we present an update of this analysis by using data with increased statistics, originating both from a larger data set including more recent measurements and by using a larger fiducial area. In addition, optimized selection criteria for enhancing light primaries are applied. We find a spectral feature for light elements, namely, a hardening at $E={10}^{17.08\pm 0.08}\mathrm{eV}$ with a change of the power law index from $-3.25\pm 0.05$ to $-2.79\pm 0.08$.

Figure 1

Layout of the KASCADE-Grande experiment. The dashed line marks the fiducial area used in Ref. [8]. The area used in this analysis is indicated by the dotted line. The experiment is located at KIT in Karlsruhe. The Grande stations (rectangles) had to be arranged to fit between the buildings. Therefore the stations are irregularly distributed and the missing station at around $(-600,-100)$ is the reason for the rectangular cut between $-80\mathrm{m}\le Y\le -200\mathrm{m}$. The original KASCADE array is located in the upper right corner. A comprehensive description of the detector and the reconstruction procedures can be found in Ref. [7].

Figure 2

The mean values of $k$ over the reconstructed energy are shown for events simulated with QGSJet-II-2, and with zenith angles between 0°–24°, and five different primaries. The distributions of $k$ over $E$ are similar in adjacent zenith angle intervals. By combining two angular bins, we increase simulation statistics. The increase of systematic uncertainties is taken into account. For comparison, the measured data (empty squares) are also shown (shifted from the bin center to the right for better visibility of the error bars). The error bars represent the rms for the measured data and the error of the mean for the simulated data. The dotted line was used in Ref. [8] to separate the events into a light and a heavy component. For an enhanced light selection on cosmic rays, the continuous line is taken for the separation, where the dashed lines depict the uncertainty of the separation, taking into account also the reconstruction uncertainty of $k$.

Figure 3

Shown are simulated and reconstructed all-particle energy spectra using QGSJet-II and EPOS-1.99 as hadronic interaction models, where for EPOS a smaller number of events are available. For the simulations, a composition of five elements (H, He, CNO, Si, and Fe) equally abundant has been used. The combined proton and helium component for QGSJet-II and the pure hydrogen component for EPOS are also displayed. In addition, reconstructed spectra for the light component (separation between He and CNO) are shown, where in both cases the QGSJet based reconstruction and selection criteria are applied. In the case of QGSJet-II the light component is reproduced, whereas for EPOS simulations the reconstructed light component shows an agreement with the initial pure proton spectrum.

Figure 4

The all-particle and electron-rich spectra from the analysis [8] in comparison to the results of this analysis with higher statistics. In addition to the light and heavy spectrum based on the separation between He and CNO, the light spectrum based on the separation on He is also shown. The error bars show the statistical uncertainties.

Figure 5

The reconstructed energy spectrum of the light mass component of cosmic rays. The number of events per energy bin is indicated as well as the range of systematic uncertainty. The error bars show the statistical uncertainties.