Analysis and implications of the spatio-spectral morphology of the Fermi Bubbles

TL;DR

This study performs a detailed pixel-by-pixel spectral analysis of the Fermi Bubbles, revealing that both hadronic and leptonic models can explain the gamma-ray emission, with implications for their origin and cosmic ray populations.

Contribution

It provides the first detailed spectral shape and normalization analysis of the Fermi Bubbles using a template-free reconstruction over ten years of data, highlighting the characteristics of the cosmic ray populations involved.

Findings

Both hadronic and leptonic models fit the data well.

Cosmic ray spectra have nearly constant break or cutoff energies with latitude.

Leptonic models require increasing electron energy density with distance from the Galactic plane.

Abstract

The Fermi Bubbles are gamma-ray structures extending from the center of the Milky Way to +/-50 degree Galactic latitude that were discovered in data obtained by the Fermi/LAT instrument. Their origin and power source remain uncertain. To help address this uncertainty, here we use a template-free reconstruction of ten years of all-sky Fermi/LAT data provided by Platz et al. (2023) to carry out a pixel-by-pixel spectral analysis of the Bubbles. We recover the position-dependent spectral shape and normalization that would be required for parent proton or electron cosmic ray populations to produce the Bubbles' observed gamma-ray spectra. We find that models in which the gamma-ray emission is driven by either hadronic or leptonic processes can explain the data equally well. The cosmic ray population driving the emission must have either broken power-law or exponentially cut-off spectra, with break or cutoff energies that are almost constant with latitude but spectral indices below the break that harden towards the Bubbles' southern tip. For the leptonic channel, reproducing the observed position-dependent gamma-ray spectrum also requires a cosmic ray electron energy density that grows with distance from the Galactic plane and increases towards the edges of the Bubbles. This finding disfavors scenarios for the origin of the Bubbles where a population of cosmic ray electrons is accelerated near the Milky Way center and subsequently advected out to the extremities of the Bubbles.