Multi-Component Imaging of the Fermi Gamma-ray Sky in the Spatio-spectral Domain

TL;DR

This paper presents advanced Bayesian methods for detailed, multi-component spatio-spectral reconstruction of the gamma-ray sky using Fermi LAT data, revealing new insights into Galactic emission structures.

Contribution

It introduces two novel Bayesian models for gamma-ray sky reconstruction, one template-free and one physics-informed, differentiating emission origins and providing high-fidelity spatial and spectral decompositions.

Findings

Revealed detailed Galactic leptonic diffuse emission structures.

Identified a gamma-ray arc coincident with an X-ray arc in the Fermi bubbles.

Provided publicly available sky reconstructions with uncertainty quantification.

Abstract

We perform two distinct spatio-spectral reconstructions of the gamma-ray sky in the range of 0.56-316 GeV based on Fermi Large Area Telescope (LAT) data. Both describe the sky brightness to be composed of a diffuse-emission and a point-source component. The first model requires minimal assumptions and provides a template-free reconstruction as a reference. It makes use of spatial and spectral correlations to distinguish between the different components. The second model is physics-informed and further differentiates between diffuse emission of hadronic and leptonic origin. For this, we assume parametric, but spatially varying energy spectra to distinguish between the processes and use thermal Galactic dust observations to indicate the preferred sites of hadronic interactions. To account for instrumental effects we model the point-spread, the energy dispersion, and the exposure of the telescope throughout the observation. The reconstruction problem is formulated as a Bayesian inference task, that is solved by variational inference. We show decompositions of the Gamma-ray flux into diffuse and point-like emissions, and of the diffuse emissions into multiple physically motivated components. The diffuse decomposition provides an unprecedented view of the Galactic leptonic diffuse emission. It shows the Fermi bubbles and their spectral variations in high fidelity and other areas exhibiting strong cosmic ray electron contents, such as a thick disk in the inner Galaxy and outflow regions. Furthermore, we report a hard spectrum gamma ray arc in the northern outer bubble co-spatial with the reported X-ray arc by the eROSITA collaboration. All our spatio-spectral sky reconstructions and their uncertainty quantification are publicly available.