The Cosmic-Ray Energy Spectrum between 2 PeV and 2 EeV Observed with the TALE detector in monocular mode

TL;DR

This paper reports on the measurement of the cosmic ray energy spectrum from 2 PeV to over 100 EeV using the TALE detector, revealing spectral features that inform models of cosmic ray origins and transitions.

Contribution

First measurement of the cosmic ray spectrum across a broad energy range from PeV to EeV energies using TALE's combined Cherenkov and fluorescence detection methods.

Findings

Detected a spectral steepening near 10^{17.1} eV.

Observed an ankle-like feature at 10^{16.2} eV.

Spectrum features suggest a transition from galactic to extragalactic cosmic rays.

Abstract

We report on a measurement of the cosmic ray energy spectrum by the Telescope Array Low-Energy Extension (TALE) air fluorescence detector. The TALE air fluorescence detector is also sensitive to the Cherenkov light produced by shower particles. Low energy cosmic rays, in the PeV energy range, are detectable by TALE as "Cherenkov Events". Using these events, we measure the energy spectrum from a low energy of $\sim 2$ PeV to an energy greater than 100 PeV. Above 100 PeV TALE can detect cosmic rays using air fluorescence. This allows for the extension of the measurement to energies greater than a few EeV. In this paper, we will describe the detector, explain the technique, and present results from a measurement of the spectrum using $\sim 1000$ hours of observation. The observed spectrum shows a clear steepening near $10^{17.1}$ eV, along with an ankle-like structure at $10^{16.2}$ eV. These features present important constraints on galactic cosmic rays origin and propagation models. The feature at $10^{17.1}$ eV may also mark the end of the galactic cosmic rays flux and the start of the transition to extra-galactic sources.