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 $-90^\circ \leq\delta\leq +44.8^\circ$ using data from $\sim 310{,}000$ events accrued at the Pierre Auger Observatory from an exposure of $(104{,}900\pm 3{,}100)$ km$^2\,$sr$\,$yr. 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 $5\,\sigma$. The quasi-uniformity of the energy spectrum across declinations disfavors an origin for the instep from a few distinctive sources.