[Paper Review] Diffuse TeV Gamma-Ray Emission in the H.E.S.S. Galactic Plane Survey

K. Egberts, F. Brun+6 more|arXiv (Cornell University)|Aug 1, 2013

Astrophysics and Cosmic Phenomena1 references3 citations

TL;DR

This study presents the first observational assessment of diffuse TeV gamma-ray emission using the H.E.S.S. Galactic Plane Survey, leveraging high-angular-resolution imaging atmospheric Cherenkov telescopes to isolate diffuse emission after rigorous background subtraction. The data reveal a significant excess at b ≈ −0.25° over the predicted pion-decay contribution from cosmic-ray interactions with interstellar gas, indicating substantial contributions from unresolved sources and inverse Compton scattering.

ABSTRACT

Diffuse gamma-ray emission has long been established as the most prominent feature in the GeV sky. Although the imaging atmospheric Cherenkov technique has been successful in revealing a large population of discrete TeV gamma-ray sources, a thorough investigation of diffuse emission at TeV energies is still pending. Data from the Galactic Plane Survey (GPS) obtained by the High Energy Stereoscopic System (H.E.S.S.) have now achieved a sensitivity and coverage adequate for probing signatures of diffuse emission in the energy range of ~100 GeV to a few TeV. Gamma-rays are produced in cosmic-ray interactions with the interstellar medium (aka "sea of cosmic rays") and in inverse Compton scattering on cosmic photon fields. This inevitably leads to guaranteed gamma-ray emission related to the gas content along the line-of-sight. Further contributions relate to those gamma-ray sources that fall below the current detection threshold and the aforementioned inverse Compton emission. Based on the H.E.S.S. GPS, we present the first observational assessment of diffuse TeV gamma-ray emission. The observation is compared with corresponding flux predictions based on the HI (LAB data) and CO (as a tracer of H2, NANTEN data) gas distributions. Consequences for unresolved source contributions and the anticipated level of inverse Compton emission are discussed.

Motivation & Objective

• To measure large-scale diffuse TeV gamma-ray emission in the Galactic Plane using imaging atmospheric Cherenkov telescopes.
• To assess the contribution of cosmic-ray interactions with interstellar gas (HI and H₂) to the observed diffuse emission via π⁰ decay.
• To constrain the contributions of unresolved gamma-ray sources and inverse Compton scattering on interstellar radiation fields.
• To evaluate the sensitivity and methodological challenges of background subtraction in diffuse emission measurements with limited-field-of-view instruments.
• To compare observed diffuse flux with theoretical predictions based on gas distributions and local cosmic-ray spectra.

Proposed method

• Used 1926 hours of dead-time corrected H.E.S.S. GPS data covering −75° < l < 60° and −2° < b < 2°.
• Applied model analysis with standard and hard event reconstruction cuts to optimize gamma-hadron separation.
• Implemented adaptive ring background subtraction to minimize signal contamination from unresolved sources.
• Excluded regions with γ-ray significance >4σ (or >4.5σ in neighboring bins) and within 0.2° of such sources to avoid source leakage.
• Excluded |b| < 1.2° to reduce contamination from gas-associated emission while preserving statistical power.
• Calculated expected π⁰-decay flux using LAB HI and NANTEN CO data, assuming constant X_CO and local cosmic-ray spectrum.

Experimental results

Research questions

• RQ1 To what extent does the observed diffuse TeV gamma-ray emission exceed the flux predicted from p-p interactions in the interstellar medium?
• RQ2 What fraction of the observed diffuse flux can be attributed to unresolved gamma-ray sources below the detection threshold?
• RQ3 How significant is the contribution of inverse Compton scattering by high-energy electrons to the diffuse TeV emission?
• RQ4 How do systematic effects from background subtraction impact the interpretation of diffuse emission measurements?
• RQ5 What is the spatial distribution of the excess emission relative to the Galactic Plane and gas density?

Key findings

• The H.E.S.S. data reveal a diffuse emission excess at b ≈ −0.25°, exceeding the baseline prediction from p-p interactions.
• In the region |b| < 1°, the observed diffuse flux is approximately 25% of the measured value, implying that unresolved sources contribute less than 75% of the signal.
• The predicted π⁰-decay flux from gas interactions accounts for only ~25% of the observed diffuse flux in the central Galactic Plane.
• The contribution of π⁰ decay to the total flux including sources is less than 10% in the |b| < 1° region, indicating that other components dominate.
• Inverse Compton scattering is expected to contribute significantly, especially at higher latitudes, but is subject to background subtraction and only gradients are measurable.
• The observed excess cannot be fully explained by known gas-associated emission, suggesting a substantial unresolved source population or enhanced inverse Compton emission.