Fermi bubbles as a source of cosmic rays above 10^{15} eV

TL;DR

This paper proposes that Fermi bubbles are formed by in-situ acceleration of electrons at shocks from star accretion events near Sgr A*, reaccelerating protons and explaining cosmic ray spectra beyond the knee.

Contribution

It introduces a novel model linking Fermi bubbles formation to star accretion shocks and reacceleration of cosmic rays, explaining features of the cosmic ray spectrum beyond the knee.

Findings

Fermi bubbles are likely formed by in-situ electron acceleration at shocks.

Reacceleration in Fermi bubbles accounts for cosmic rays beyond the knee.

The model matches observed cosmic ray flux and spectral indexes.

Abstract

Fermi bubbles are giant gamma-ray structures extended north and south of the Galactic center with characteristic sizes of order of 10 kpc recently discovered by Fermi Large Area Telescope. Good correlation between radio and gamma-ray emission in the region covered by Fermi bubbles implies the presence of high-energy electrons in this region. Since it is relatively difficult for relativistic electrons of this energy to travel all the way from the Galactic sources toward Fermi bubbles one can assume that they accelerated in-situ. The corresponding acceleration mechanism should also affect the distribution of the relativistic protons in the Galaxy. Since protons have much larger lifetimes the effect may even be observed near the Earth. In our model we suggest that Fermi bubbles are created by acceleration of electrons on series of shocks born due to periodic star accretions by supermassive black hole Sgr A*. We propose that hadronic CR within the "knee" of the observed CR spectrum are produced by Galactic supernova remnants distributed in the Galactic disk. Reacceleration of these particles in the Fermi Bubble produces CRs beyond the knee. This model provides a natural explanation of the observed CR flux, spectral indexes, and matching of spectra at the knee.