Features of the Energy Spectrum of Cosmic Rays above $2.5\times {10}^{18}\mathrm{eV}$ Using the Pierre Auger Observatory

We report a measurement of the energy spectrum of cosmic rays above $2.5\times {10}^{18}\mathrm{eV}$ based on 215 030 events. New results are presented: at about $1.3\times {10}^{19}\mathrm{eV}$, the spectral index changes from $2.51\pm 0.03(\mathrm{stat})\pm 0.05(\mathrm{syst})$ to $3.05\pm 0.05(\mathrm{stat})\pm 0.10(\mathrm{syst})$, evolving to $5.1\pm 0.3(\mathrm{stat})\pm 0.1(\mathrm{syst})$ beyond $5\times {10}^{19}\mathrm{eV}$, while no significant dependence of spectral features on the declination is seen in the accessible range. These features of the spectrum can be reproduced in models with energy-dependent mass composition. The energy density in cosmic rays above $5\times {10}^{18}\mathrm{eV}$ is $[5.66\pm 0.03(\mathrm{stat})\pm 1.40(\mathrm{syst})]\times {10}^{53}\mathrm{erg}{\mathrm{Mpc}}^{-3}$.

Figure 1

Top: energy spectrum scaled by $E^2$ with the number of detected events in each energy bin. In this representation the data provide an estimation of the differential energy density per decade. Bottom: energy spectrum scaled by $E^3$ fitted with a sequence of four power laws (red line). The numbers ($i=1,\dots ,4$) enclosed in the circles identify the energy intervals where the spectrum is described by a power law with spectral index ${\gamma }_i$. The shaded band indicates the statistical uncertainty of the fit. Upper limits are at the 90% confidence level.

Figure 2

Energy density obtained with the best fit parameters of the benchmark scenario used for illustration, as described in the text. The dashed curve shows the energy range that is not used in the fit and where an additional component is needed for describing the spectrum.