The Fermi bubbles from stochastic acceleration of electrons in a Galactic outflow

TL;DR

This paper models the stochastic acceleration of electrons in the Galactic outflow to explain the Fermi bubbles, providing a detailed numerical solution that incorporates acceleration, transport, and energy losses, fitting observational data.

Contribution

It presents the first detailed numerical solution of electron acceleration in the Fermi bubbles, including stochastic and shock acceleration mechanisms, fitting observational spectra and brightness.

Findings

Transport coefficients constrained by observations

Stochastic acceleration explains the electron energy distribution

Shock acceleration at the bubble's edge contributes to the spectrum

Abstract

The discovery of the Fermi bubbles---a huge bilobular structure seen in GeV gamma-rays above and below the Galactic center---implies the presence of a large reservoir of high energy particles at $\sim 10 \, \text{kpc}$ from the disk. The absence of evidence for a strong shock coinciding with the edge of the bubbles, and constraints from multi-wavelength observations point towards stochastic acceleration by turbulence as a likely mechanism of acceleration. We have investigated the time-dependent acceleration of electrons in a large-scale outflow from the Galactic centre. For the first time, we present a detailed numerical solution of the particle kinetic equation that includes the acceleration, transport and relevant energy loss processes. We also take into account the addition of shock acceleration of electrons at the bubble's blast wave. Fitting to the observed spectrum and surface brightness distribution of the bubbles allows determining the transport coefficients, thereby shedding light on the origin of the Fermi bubbles.