Diffuse Galactic Gamma-ray Emission with H.E.S.S.

Abstract

While imaging atmospheric Cherenkov telescopes have been very successful in the discovery and detailed measurement of γ-ray sources, observation of large-scale diffuse emission has been complicated by their small field of view. With the H.E.S.S. Galactic plane survey exists for the first time a deep-exposure data set of large spatial extension, covering the central part of the Milky Way. In the analysis of these data, a signal of large-scale γ-ray emission along the Galactic plane is detected in regions off significantly detected γ-ray sources. This is the first time that large-scale γ-ray emission is observed by imaging atmospheric Cherenkov telescopes. Due to the imperfect γ-hadron separation and the applied background subtraction, the analysis is only sensitive to signals of extensions in Galactic latitude that are small compared to the H.E.S.S. field of view. Emission with larger scale height can be recovered only partly. Contributions to the observed signal originate probably from unresolved γ-ray sources and a truly diffuse emission of hadronic interactions with π 0 decay and inverse Compton scattering. Calculations show that the minimum γ-ray emission from π 0 -decay represents already a significant fraction of the total signal. While unresolved sources provide the smallest scale height and are recoverable to a large extend, the hadronic emission via π 0 decay follows the broader gas distribution and looses around a third of its signal. The inverse Compton component depends at TeV γ-ray energies on the short lifetime and propagation distances of the emitting electrons and renders predictions difficult. At lower energies a smooth emission with large scale height results in a signal loss of ∼ 95%. XI Multifrequency Behaviour of High Energy Cosmic Sources Workshop