Diffuse Galactic gamma-ray emission with H.E.S.S

TL;DR

This paper reports the first detection of large-scale diffuse gamma-ray emission along the Galactic Plane at TeV energies using H.E.S.S., highlighting contributions from cosmic-ray interactions, unresolved sources, and neutral pion decay.

Contribution

It presents the first observation of large-scale diffuse gamma-ray emission at TeV energies with imaging atmospheric Cherenkov telescopes, expanding understanding beyond previous GeV energy studies.

Findings

Detection of characteristic excess gamma-ray emission along the Galactic Plane.

Large-scale gamma-ray emission is a mix of diffuse emission and unresolved sources.

Neutral pion decay significantly contributes to the observed gamma-ray flux.

Abstract

Diffuse $\gamma$-ray emission is the most prominent observable signature of celestial cosmic-ray interactions at high energies. While already being investigated at GeV energies over several decades, assessments of diffuse $\gamma$-ray emission at TeV energies remain sparse. After completion of the systematic survey of the inner Galaxy, the H.E.S.S. experiment is in a prime position to observe large-scale diffuse emission at TeV energies. Data of the H.E.S.S. Galactic Plane Survey are investigated in regions off known $\gamma$-ray sources. Corresponding $\gamma$-ray flux measurements were made over an extensive grid of celestial locations. Longitudinal and latitudinal profiles of the observed $\gamma$-ray fluxes show characteristic excess emission not attributable to known $\gamma$-ray sources. For the first time large-scale $\gamma$-ray emission along the Galactic Plane using imaging atmospheric Cherenkov telescopes has been observed. While the background subtraction technique limits the ability to recover modest variation on the scale of the H.E.S.S. field of view or larger, which is characteristic of the inverse Compton scatter-induced Galactic diffuse emission, contributions of neutral pion decay as well as emission from unresolved $\gamma$-ray sources can be recovered in the observed signal to a large fraction. Calculations show that the minimum $\gamma$-ray emission from $\pi^0$-decay represents a significant contribution to the total signal. This detection is interpreted as a mix of diffuse Galactic $\gamma$-ray emission and unresolved sources.