Measurement of the cosmic ray energy spectrum with IceTop-73

We report on the measurement of the all-particle cosmic ray energy spectrum with the IceTop air shower array in the energy range from 1.58 PeV to 1.26 EeV. The IceTop air shower array is the surface component of the IceCube Neutrino Observatory at the geographical South Pole. The analysis was performed using only information from IceTop. The data used in this work were taken from June 1, 2010 to May 13, 2011. During that period the IceTop array consisted of 73 stations, compared to 81 in its final configuration. The measured spectrum exhibits a clear deviation from a single power law above the knee around 4 PeV and below 1 EeV. We observe spectral hardening around 18 PeV and steepening around 130 PeV.

Figure 1

Surface map of IceTop in 2010. The polygon represents the containment region ($577,265{\mathrm{m}}^2$).

Figure 2

Fractional composition of the H4a model in four elemental groups. CNO and MgSi groups were combined due to lack of Mg and Si simulation.

Figure 3

${\mathrm{S}}_{125}$ spectrum. Different histograms represent event selection by number of triggered stations. All cuts from Sec. 4b, with the exception of ${log}_{10}(S_{125})\ge 0.0$, were applied.

Figure 4

${log}_{10}(S_{125})$ vs ${log}_{10}(E_{\mathrm{true}})$ scatter plot for proton primary simulation with $\cos \theta \ge 0.95$, weighted by a flux model $\frac{dN}{dE}\propto E^{-2.7}$.

Figure 5

Examples of the true energy distributions for four $S_{125}$ slices fitted by Gaussian functions for comparison. The events were simulated using pure protons with $\frac{dN}{dE}\propto E^{-2.7}$. Shower size for the four examples are $0.3<{log}_{10}(S_{125})\le 0.35$, $0.8<{log}_{10}(S_{125})\le 0.85$, $1.3<{log}_{10}(S_{125})\le 1.35$, $1.8<{log}_{10}(S_{125})\le 1.85$. The zenith range is $\cos \theta \ge 0.95$.

Figure 6

$S_{125}$-to-$E_{\mathrm{true}}$ relations in four zenith ranges for three composition assumptions.

Figure 7

Energy resolution as a function of reconstructed energy for H4a assumption in two zenith bins.

Figure 8

Energy bias as a function of reconstructed energy for H4a assumption in two zenith bins.

Figure 9

Effective area for H4a composition with $\cos \theta \ge 0.8$.

Figure 10

The difference in the spectra obtained using SIBYLL 2.1 or QGSJet-II as an interaction model for IceTop and KASCADE-Grande [5, 22].

Figure 11

IceTop-73 cosmic ray energy spectrum with five composition assumptions and $\cos \theta \ge 0.80$.

Figure 12

$S_{125}$-to-$E_{\mathrm{true}}$ relations for four composition assumptions in the $0.85\le \cos \theta <0.90$ zenith range.

Figure 13

Cosmic ray energy spectrum for three composition assumptions and four zenith ranges.

Figure 14

Spectral fits in different energy ranges.

Figure 15

IceTop-73 spectrum in comparison to other recent (within the last six years) experiments: KASCADE [21], KASCADE-Grande [22], Tunka-133 [7], GAMMA [3], Tibet [23]. Errors are statistical only.

Figure 16

IceTop-73 spectrum in comparison to previous IceCube results [1, 2]. Errors bars are systematic errors added in quadrature.

Figure 17

IceTop-73 snow cover in meters. The dashed polygon represents the new, less snowy part of the detector; the solid polygon shows the old, snowy part of the detector.

Figure 18

Shower size spectra for two containment cuts weighted by their respective areas with and without snow correction.

Figure 19

Shower core distribution for one month of data, $\cos \theta \ge 0.95$, with five or more stations triggered, ${log}_{10}(S_{125}/\mathrm{VEM})>0.0$.