A Novel Analytical Model of the Magnetic Field Configuration in the Galactic Center Explaining the Diffuse Gamma-Ray Emission (ICRC-2019) (1909.08379v1)

Abstract: Reliable identification of the origin of the high-energy non-thermal emission from the Galactic Center (GC) is not achievable without adequate consideration of the ambient conditions such as the magnetic field configuration or gas distribution. In a first step, we present a model that can explain the diffuse gamma-ray emission as measured by H.E.S.S. for small longitudes in the Galactic Center region but comes to grief with higher longitudes. The model is given via the solution of a transport equation that allows for a radial dependency of the mass distribution. In order to move from this semi-analytical approximation toward a full understanding of the PeVatron signature, we present a new 3D analytical model of the gas distribution in the Central Molecular Zone (CMZ). Furthermore, we derive for the first time a 3D model of the magnetic field configuration and strength in the CMZ, which is analytical and divergence-free. The model is built via a combination of a model for the diffuse inter-cloud medium, local molecular clouds and non-thermal filaments at which local information are based on investigations from previous works and the molecular gas density. It can be shown that without an efficient longitudinal CR entrapment, a single source at the center does not facilely suffice the diffuse gamma-ray detection. Further, we show that the new magnetic field model GBFD19 is compatible with recent polarization data and has a significant impact on the longitudinal profiles of CR propagation.

• The paper introduces a novel 3D analytical model that maps the Galactic Center's magnetic field, enhancing our understanding of cosmic ray confinement.
• It integrates a detailed gas distribution framework across molecular clouds, the central region, and the intercloud medium to match observed CMZ properties.
• By employing CRPropa simulations, the study reveals improved cosmic ray entrapment effects that better explain the region's diffuse gamma-ray emission.

Analytical Model of Magnetic Field Configuration in the Galactic Center

The paper introduces an analytical model designed to elucidate the configuration of the magnetic field in the Galactic Center (GC) and its role in explaining the diffuse gamma-ray emission observed in the region. This work is positioned at the intersection of astrophysical observations, theoretical modeling, and cosmic ray (CR) propagation studies. The authors present a methodologically rigorous approach centered on addressing the limitations of existing models in representing the ambient conditions of the GC, particularly focusing on the magnetic field and gas distribution within the Central Molecular Zone (CMZ).

The gamma-ray emission from the GC, especially as captured by instruments like H.E.S.S., has long suggested a spectrum implicating CR phenomena, but the exact source of these emissions remains contested. Previous approaches have often struggled to explain the emission characteristics across varying longitudes. Central to resolving this issue is the understanding of CR entrapment and the role of the local magnetic field configurations, which this paper provides in detail through a novel 3D analytical model of both the gas and magnetic fields in the CMZ.

Gas Distribution Model

In the ongoing pursuit to better represent the ambient conditions influencing CR propagation, the paper outlines a 3-component model for gas distribution within the CMZ:

1. Molecular Clouds (MCs): The model bases its parameters on known local structure densities, extending to recognized entities like Sgr C and Sgr D.
2. Inner Central Region: Leveraging data specific to the inner 10 pc, this component integrates well-documented central cluster characteristics.
3. Intercloud Medium (ICM): Here, the authors adjust parameters to fill gaps left by previous works, ensuring a comprehensive portrayal of diffuse intercloud matter.

Magnetic Field Configuration

Addressing a critical gap in existing literature, the authors propose a new model to describe the GC's magnetic field, integrating insights from several sources:

• The paper's analytical magnetic field model is built upon a divergence-free structure, incorporating poloidal fields in both the intercloud and non-thermal filament regions and predominantly horizontal fields within molecular clouds.
• Fitting existing polarization data available for the GC informs this newly configured model, resulting in demonstrably higher accuracy in reflecting observational data.

Numerical Results and Implications

The findings highlight that the existing models' oversights regarding the GC's magnetic field may have caused them to underestimate CR confinement, impacting our understanding of diffuse gamma-ray distributions. By applying their model to CR propagation simulations, notably using the CRPropa tool, the authors demonstrate a more pronounced entrapment effect within the longitudinal profile of CR propagation than previously accounted for—a significant result for both theoretical and observational astrophysics.

Implications and Future Directions

The presented model represents a robust step towards reconciling empirical data with CR propagation theories, offering a toolset for deeper exploration of CR-generated phenomena in the GC. This has immediate implications for both observational strategies in gamma-ray astronomy and the theoretical frameworks underpinning CR propagation studies.

While the model significantly enhances our understanding of the GC's magnetic environment, the authors suggest further simulations of gamma-ray production against this refined environmental backdrop as a logical continuation of their work. A comprehensive validation against future observational data sets will likely drive further refinements and adaptations, potentially uncovering new aspects of galactic dynamics and CR origins.

In conclusion, this paper contributes a detailed 3D model with clear applications and implications for the broader astrophysical community, setting the stage for nuanced investigations into the high-energy phenomena observed in the Galactic Center.