Separation of γ-Ray, Electron and Proton induced Air Showers applied to Diffuse Emission Studies with H.E.S.S.

A fundamental issue in ground-based gamma-ray astronomy is the identification of γ-ray events among the overwhelming background of air showers induced by charged cosmic rays. Reconstruction techniques exist to distinguish most of the background of hadrons but an irreducible background of electrons and gamma-like protons still remain. I present here a new technique making use of high-altitude Cherenkov light emitted by the charged primary particle and air shower development properties. This method provides a way to distinguish between electrons and gamma rays on a statistical basis. In addition to this, the remaining proton background can also be identified. The technique was developed, tested and applied to studies using the High Energy Stereoscopic System (H.E.S.S.) located in Namibia. The analysis method is especially important in the detection of diffuse signals and eliminates the necessity of a background region in the field of view. The technique was applied to three scientific studies. The latitude profile of the Galactic diffuse gamma-ray emission was analysed. A width of σ = 0.25±0.05 deg (0.20±0.06 deg) for energies of 380 to 900 GeV (1 to 6 TeV) was determined. The cosmic electron spectrum was measured between 0.38 and 14 TeV and a broken power law was fit to the data. The spectrum steepens from an index of 3.08±0.06 to 3.72±0.12 at a break in energy of 1.11±0.04 TeV. In addition, upper limits on the maximum y-ray contamination from the Isotropic γ-Ray Background was placed at 4 x 10^3 (5 x 10^3) MeV cm^-2 s^-1 sr^-1 for energies of 1 to 6 TeV (380 to 900 GeV)