Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Features of the energy spectrum of cosmic rays above $2.5{\times}
  10^{18}$ eV using the Pierre Auger Observatory   We report a measurement of the energy spectrum of cosmic rays above
$2.5{\times} 10^{18}$ eV based on $215,030$ events. New results are presented:
at about $1.3{\times} 10^{19}$ eV, the spectral index changes from $2.51 \pm
0.03 \textrm{ (stat.)} \pm 0.05 \textrm{ (sys.)}$ to $3.05 \pm 0.05 \textrm{
(stat.)}\pm 0.10\textrm{ (sys.)}$, evolving to $5.1\pm0.3\textrm{ (stat.)} \pm
0.1\textrm{ (sys.)}$ beyond $5{\times} 10^{19}$ 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
$5{\times} 10^{18}$ eV is $(5.66 \pm 0.03 \textrm{ (stat.)} \pm 1.40 \textrm{
(sys.)} ) {\times} 10^{53}~$erg Mpc$^{-3}$.

23
The categories of this paper are: solv-int
32
The categories of this paper are: atom-ph
14
The categories of this paper are: math-ph
21
The categories of this paper are: adap-org
25
The categories of this paper are: patt-sol
10
The categories of this paper are: q-bio
34
The categories of this paper are: chem-ph
7
The categories of this paper are: hep-th
9
The categories of this paper are: nlin
0
The categories of this paper are: hep-ph
12
The categories of this paper are: nucl-th
26
The categories of this paper are: bayes-an
24
The categories of this paper are: dg-ga
5
The categories of this paper are: gr-qc
35
The categories of this paper are: mtrl-th
4
The categories of this paper are: cond-mat