Multi-wavelength Emission from the Fermi Bubble III. Stochastic (Fermi) Re-Acceleration of Relativistic Electrons Emitted by SNRs

TL;DR

This paper investigates stochastic re-acceleration of electrons emitted by supernova remnants in the Galactic disk to explain the nonthermal radio and gamma-ray emissions from the Fermi Bubbles, emphasizing the role of turbulence and energy losses.

Contribution

It introduces a model where SNR-emitted electrons are re-accelerated in the Galactic halo by turbulence, accounting for observed emissions and reconciling radio spectrum discrepancies with adiabatic losses.

Findings

Re-acceleration up to 10^12 eV explains gamma-ray emissions.

Model gamma spectrum aligns with Fermi data.

Including plasma outflows improves radio spectrum fit.

Abstract

We analyse the model of stochastic re-acceleration of electrons, which are emitted by supernova remnants (SNRs) in the Galactic Disk and propagate then into the Galactic halo, in order to explain the origin on nonthermal (radio and gamma-ray) emission from the Fermi Bubbles (FB). We assume that the energy for re-acceleration in the halo is supplied by shocks generated by processes of star accretion onto the central black hole. Numerical simulations show that regions with strong turbulence (places for electron re-acceleration) are located high up in the Galactic Halo about several kpc above the disk. The energy of SNR electrons that reach these regions does not exceed several GeV because of synchrotron and inverse Compton energy losses. At appropriate parameters of re-acceleration these electrons can be re-accelerated up to the energy 10E12 eV which explains in this model the origin of the observed radio and gamma-ray emission from the FB. However although the model gamma-ray spectrum is consistent with the Fermi results, the model radio spectrum is steeper than the observed by WMAP and Planck. If adiabatic losses due to plasma outflow from the Galactic central regions are taken into account, then the re-acceleration model nicely reproduces the Planck datapoints.