Multi-wavelength Emission from the Fermi Bubble II. Secondary Electrons and the Hadronic Model of the Bubble

TL;DR

This paper investigates the gamma-ray and radio emissions from the Fermi Bubbles using a mixed hadronic and leptonic model, analyzing the roles of secondary electrons, magnetic fields, and plasma outflows.

Contribution

It demonstrates that a combined hadronic and leptonic model explains the Fermi Bubbles' emissions, constraining magnetic field strengths and highlighting the importance of primary electrons.

Findings

Gamma-ray flux can be up to 80% from proton collisions.

Magnetic field strength is constrained to 2-7 μG for the model to fit data.

Pure hadronic models require high magnetic fields and CR power, inconsistent with observations.

Abstract

We analyse the origin of the gamma-ray flux from the Fermi Bubbles (FBs) in the framework of the hadronic model in which gamma-rays are produced by collisions of relativistic protons with the protons of background plasma in the Galactic halo. It is assumed in this model that the observed radio emission from the FBs is due to synchrotron radiation of secondary electrons produced by $pp$ collisions. However, if these electrons loose their energy by the synchrotron and inverse-Compton, the spectrum of secondary electrons is too soft, and an additional arbitrary component of primary electrons is necessary in order to reproduce the radio data. Thus, a mixture of the hadronic and leptonic models is required for the observed radio flux. It was shown that if the spectrum of primary electrons is $\propto E_e^{-2}$, the permitted range of the magnetic field strength is within 2 - 7 $\mu$G region. The fraction of gamma-rays produced by $pp$ collisions can reach about 80% of the total gamma-ray flux from the FBs. If magnetic field is <2 $\mu$G or >7 $\mu$G the model is unable to reproduce the data. Alternatively, the electrons in the FBs may lose their energy by adiabatic energy losses if there is a strong plasma outflow in the GC. Then, the pure hadronic model is able to reproduce characteristics of the radio and gamma-ray flux from the FBs. However, in this case the required magnetic field strength in the FBs and the power of CR sources are much higher than those followed from observations.