The Energy Spectrum of Ultra-High Energy Cosmic Rays across Declinations $-90^{\circ }$ to $+44.8^{\circ }$ as measured at the Pierre Auger Observatory

Abstract

The energy spectrum of cosmic rays above 2.5 EeV has been measured across the declination range $-90^{\circ }\le \hat{I}´\le +44.8^{\circ }$ using data from $\sim 310,000$ events accrued at the Pierre Auger Observatory from an exposure of $(104,900\pm 3,100)$ km${}^2$sryr. No significant variations of energy spectra with declination are observed, after allowing or not for non-uniformities across the sky arising from the well-established dipolar anisotropies in the arrival directions of ultra-high energy cosmic rays. Additionally, the instep feature in the spectrum at $\simeq$ 10 EeV reported previously is now established at a significance above $5\ddot{I}ƒ$. The quasi-uniformity of the energy spectrum across declinations disfavors an origin for the instep from a few distinctive sources.

17 pages, 4 figures, version accepted for publication in Physical Review Letters