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HESS - Abteilung Hofmann
Abstract:
A fundamental issue in ground-based gamma-ray astronomy is the identification
of gamma-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° (0.20 ± 0.06_) 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 Γ = 3.08 ± 0.06 to Γ =
3.72 ± 0.12 at a break in energy of 1.11 ± 0.04 TeV. In addition, upper
limits on the maximum -ray contamination from the Isotropic -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).
