Fermi-LAT Observations of High-Energy Gamma-Ray Emission Toward the Galactic Center (1511.02938v1)

Abstract: The Fermi Large Area Telescope (LAT) has provided the most detailed view to date of the emission towards the Galactic centre (GC) in high-energy gamma-rays. This paper describes the analysis of data taken during the first 62 months of the mission in the energy range 1-100 GeV from a $15^\circ \times 15^\circ$ region about the direction of the GC, and implications for the interstellar emissions produced by cosmic ray (CR) particles interacting with the gas and radiation fields in the inner Galaxy and for the point sources detected. Specialised interstellar emission models (IEMs) are constructed that enable separation of the gamma-ray emission from the inner $\sim 1$ kpc about the GC from the fore- and background emission from the Galaxy. Based on these models, the interstellar emission from CR electrons interacting with the interstellar radiation field via the inverse Compton (IC) process and CR nuclei inelastically scattering off the gas producing gamma-rays via $\pi^0$ decays from the inner $\sim 1$ kpc is determined. The IC contribution is found to be dominant in the region and strongly enhanced compared to previous studies. A catalog of point sources for the $15^\circ \times 15^\circ$ region is self-consistently constructed using these IEMs: the First Fermi-LAT Inner Galaxy point source Catalog (1FIG). After subtracting the interstellar emission and point-source contributions from the data a residual is found that is a sub-dominant fraction of the total flux. If spatial templates that peak toward the GC are used to model the positive residual and included in the total model for the $15^\circ \times 15^\circ$ region, the agreement with the data improves, but none of the additional templates account for all of the residual structure. The spectrum of the positive residual modelled with these templates has a strong dependence on the choice of IEM. [Abridged]

• The paper demonstrates that inverse Compton scattering is the dominant process in the Galactic Center, challenging previous gamma-ray production assumptions.
• It employs detailed interstellar emission models and refined point source catalogs to differentiate between diffuse and discrete gamma-ray sources.
• Residual emission above 2 GeV suggests complex astrophysical processes, hinting at potential dark matter signals and necessitating further investigation.

Analysis of High-Energy Gamma-Ray Emission in the Galactic Centre

The detailed paper of high-energy gamma-ray emissions from the Galactic Centre (GC) is crucial for understanding cosmic ray interactions and point source distributions in this complex and bright region, which houses both known cosmic ray sources and a supermassive black hole. This paper focuses on observations gathered by the Large Area Telescope (LAT) during the first 62 months of the Fermi Gamma-ray Space Telescope mission, specifically targeting energies from 1 to 100 GeV within a 15° by 15° region centered on the GC. The paper models interstellar emissions and examines point source catalogs to interpret the gamma-ray data.

Interstellar Emission Modelling

The analysis utilizes various Interstellar Emission Models (IEMs) to separate gamma-ray emission from the GC from foreground and background interstellar processes. These models are crucial because the emission from cosmic rays interacting with interstellar gas creates a significant background, complicating the identification of discrete sources. Two primary cosmic ray source distribution models—Pulsars and OB-stars—are employed, representing different scenarios of cosmic ray distribution throughout the Galaxy.

Key findings show that the inverse Compton (IC) scattering is the dominant process for gamma-ray production in the inner 1 kpc, a significant deviation from previous assumptions where neutral gas interactions were presumed equally contributive. The models indicate a much-enhanced IC contribution, hinting at higher cosmic ray electron densities or more intense interstellar radiation fields than previously modeled.

Point Source Catalog Construction

The First Inner Galaxy point source Catalog (1FIG), developed by employing wavelet-based algorithms and maximum-likelihood fitting, highlights the need for specific modeling approaches in regions of high interstellar emission complexity. The catalog contains point sources identified within the paper region, which are compared to existing catalogs such as the Third Source Catalog (3FGL). Notably, the source density differs between catalogs, highlighting the impact of model variations on point source detection.

Residual Emission and Dark Matter Hypotheses

Despite accounting for interstellar emission and cataloged point sources, a residual emission persists, particularly pronounced above 2 GeV. Several spatial templates, including those modeling dark matter annihilation and decay scenarios, are tested to account for this excess. While an improved fit is achieved with these templates, they do not wholly resolve the residual structure, pointing to complex underlying processes. Notably, the spectral characteristics of residual emission are highly dependent on the chosen interstellar emission model, underscoring the challenge in pinpointing its exact nature.

Implications for Future Research

The analysis of the gamma-ray emissions towards the GC offers significant insights into the astrophysical mechanisms at play and poses several implications for future research. With enhanced IC contributions, models of cosmic ray dynamics and interstellar radiation field distributions may need revisiting and further refinement in theoretical frameworks. While the paper does not definitively ascribe the gamma-ray excess to a singular origin, continuing observations and refined models might enhance our understanding of potential dark matter interactions and unknown astrophysical processes.

Overall, this research underscores the complexity of analyzing high-energy emissions in the GC, demonstrating the necessity of comprehensive modeling and careful interpretation of both diffuse and point-like gamma-ray sources for advancing our understanding of the cosmic environment surrounding the supermassive black hole at our Galaxy's core. Future work, possibly extending the scope of current cosmic ray propagation models and exploring more detailed three-dimensional interstellar medium distributions, will be essential.