Search for EeV Protons of Galactic Origin

TL;DR

This study searches for galactic-origin protons in the EeV energy range by analyzing anisotropy in cosmic ray arrival directions, finding no significant anisotropy and setting an upper limit on galactic contribution.

Contribution

First to use Telescope Array data to constrain the fraction of galactic protons at EeV energies through anisotropy analysis.

Findings

No anisotropy detected in galactic plane or anticenter directions.

Upper limit of 1.3% on galactic cosmic ray fraction at 95% confidence.

Data consistent with isotropic distribution of cosmic rays.

Abstract

Cosmic rays in the energy range $10^{18.0}$ - $10^{18.5}$ eV are thought to have a light, probably protonic, composition. To study their origin one can search for anisotropy in their arrival directions. Extragalactic cosmic rays should be isotropic, but galactic cosmic rays of this type should be seen mostly along the galactic plane, and there should be a shortage of events coming from directions near the galactic anticenter. This is due to the fact that, under the influence of the galactic magnetic field, the transition from ballistic to diffusive behavior is well advanced, and this qualitative picture persists over the whole energy range. Guided by models of the galactic magnetic field that indicate that the enhancement along the galactic plane should have a standard deviation of about 20$^\circ$ in galactic latitude, and the deficit in the galactic anticenter direction should have a standard deviation of about 50$^\circ$ in galactic longitude, we use the data of the Telescope Array surface detector in $10^{18.0}$ to $10^{18.5}$ eV energy range to search for these effects. The data are isotropic. Neither an enhancement along the galactic plane nor a deficit in the galactic anticenter direction is found. Using these data we place an upper limit on the fraction of EeV cosmic rays of galactic origin at 1.3% at 95% confidence level.