[Paper Review] Study of the VHE diffuse emission in the central 200 pc of our Galaxy with H.E.S.S

A. Lemière, R. Terrier+3 more|arXiv (Cornell University)|Oct 15, 2015

Astrophysics and Cosmic Phenomena3 citations

TL;DR

This study reanalyzes the very high energy (VHE) diffuse gamma-ray emission in the central 200 pc of the Milky Way using 259 hours of H.E.S.S. data and a 2D maximum likelihood fitting method. It confirms that ~50% of the ridge emission correlates with dense gas tracers, reveals a large-scale diffuse component not linked to dense gas, and detects a new source, HESS J1746−285, spatially coincident with a pulsar wind nebula candidate, suggesting a radial cosmic-ray gradient possibly driven by the central black hole.

ABSTRACT

The very high energy emission from the Galactic Center Ridge was revealed by the High Energy Stereoscopic System (H.E.S.S.) in 2006, after subtraction of the point sources HESS J1745-290, possibly associated with Sgr A$^\star$, and HESS J1747$-$281, associated with the composite supernova remnant G0.9$+$0.1. The hard spectrum of the Ridge emission and its spatial correlation with the local gas density suggest that the emission is due to collisions of multi-TeV cosmic rays with the dense clouds of interstellar gas present in this region. The much larger H.E.S.S. dataset (250 hrs) that is now available from this region and the improved analysis method dedicated to the detection of faint emission allow us to reconsider the characterization of this gamma-ray emission in the central 200 pc of our Galaxy through a detailed morphology study. To test the various contributions to the total gamma-ray emission, we use a 2D maximum likelihood approach that allows to constrain a phenomenological model of the signal. We discuss the nature of the various components, and their implication on the cosmic-ray distribution in the central region of our Galaxy. Finally, we will reveal an additional source in this region and will discuss its potential nature.

Motivation & Objective

• To re-characterize the morphology and origin of the VHE diffuse gamma-ray emission in the central 200 pc of the Milky Way using a larger dataset and improved analysis techniques.
• To test whether the emission is primarily due to hadronic cosmic rays interacting with dense molecular gas, as previously suggested.
• To identify and characterize additional components in the gamma-ray emission that do not correlate with dense gas tracers, such as unresolved sources or diffuse gas phases.
• To investigate the presence of a central excess component and its implications for cosmic-ray acceleration in the Central Molecular Zone.

Proposed method

• A 2D maximum likelihood fitting technique is applied to gamma-ray images to model the spatial distribution of emission, allowing for precise morphological decomposition.
• The analysis uses a multivariate analysis procedure within the HAP-Fr pipeline to enhance signal-to-background separation and improve angular resolution to R68% = 0.077°.
• Background is estimated using the reflected region method with off-source regions matched in size and offset to on-source regions to minimize systematic errors.
• Gas tracers such as CS and HCN are used as templates to compare with the gamma-ray morphology, and a Gaussian smoothing of 0.8° is applied to reconcile discrepancies in the gas and gamma-ray profiles.
• Monte Carlo simulations with 100 Poissonian realizations are used to validate the significance of residual maps and detect potential new sources.
• A phenomenological model is constructed with multiple components: a ridge component correlated with dense gas, a large-scale diffuse component, a central excess, and a new point source.

Experimental results

Research questions

• RQ1 Is the VHE diffuse gamma-ray emission in the Galactic Center primarily due to hadronic interactions of cosmic rays with dense molecular gas, as previously inferred?
• RQ2 What is the origin of the large-scale diffuse emission component that does not correlate with dense gas tracers?
• RQ3 Does the central 30 pc of the Galactic Center host a radial gradient in cosmic-ray density, and what could be its physical origin?
• RQ4 Is the newly detected source HESS J1746−285 physically associated with the pulsar wind nebula candidate G0.13−0.13, and what is its nature?
• RQ5 To what extent do unresolved sources or a diffuse molecular gas phase contribute to the uncorrelated diffuse gamma-ray emission?

Key findings

• Approximately 50% of the VHE ridge emission is spatially correlated with dense gas tracers such as CS, consistent with hadronic interactions of multi-TeV cosmic rays.
• A large-scale diffuse emission component is required to fit the data, which does not correlate with dense gas tracers and may arise from unresolved sources or a diffuse molecular gas phase with ~100 cm⁻³ density.
• A central excess component is detected within ~30 pc of Sgr A⋆, with a radial extent of ~14 pc, consistent with a 1/r-like cosmic-ray gradient possibly driven by a central source such as the supermassive black hole.
• A new gamma-ray source, HESS J1746−285, is detected at a significance level consistent with a point source, spatially coincident with the Fermi Arc and the X-ray pulsar wind nebula candidate G0.13−0.13.
• The spectral index of the ridge emission is measured to be 2.3 ± 0.1, indicating a harder spectrum than the local cosmic-ray flux, supporting ongoing or recent particle acceleration in the region.
• The discrepancy between the gamma-ray and gas profiles beyond 100–150 pc is reconciled by applying a Gaussian smoothing of σ = 0.8° to the gas column density, suggesting a combination of decreasing cosmic-ray density and line-of-sight matter distribution.