Physical Review Letters ( IF 8.385 ) Pub Date :
A. Aab et al.

We report a measurement of the energy spectrum of cosmic rays above $2.5×{10}^{18}$~eV based on $215,030$ events. New results are presented: at about $1.3×{10}^{19}$~eV, the spectral index changes from $2.51±0.03\left(stat.\right)±0.05\left(sys.\right)$ to $3.05±0.05\left(stat.\right)±0.10\left(sys.\right)$, evolving to $5.1±0.3\left(stat.\right)±0.1\left(sys.\right)$ beyond $5×{10}^{19}$~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 $5×{10}^{18}$~eV is $\left(5.66±0.03\left(stat.\right)±1.40\left(sys.\right)\right)×{10}^{53}\phantom{\rule{0.222em}{0ex}}$erg Mpc${}^{-3}$.

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