Connecting Synchrotron, Cosmic Rays, and Magnetic Fields in the Plane of the Galaxy

TL;DR

This study models the Galactic magnetic field by integrating synchrotron emission data with advanced cosmic-ray electron propagation simulations, revealing a harder low-energy spectrum and challenging models with re-acceleration processes.

Contribution

It introduces a more realistic cosmic-ray electron model using GALPROP, reducing previous degeneracies and enabling analysis of the low-energy electron spectrum in the Galaxy.

Findings

Low-energy injection spectrum slope α ≈ -1.34

Predicted synchrotron spectral index ranges from -2.8 to -2.91

Re-acceleration models are incompatible with observed synchrotron data

Abstract

We extend previous work modeling the Galactic magnetic field in the plane using synchrotron emission in total and polarised intensity. In this work, we include a more realistic treatment of the cosmic-ray electrons using the GALPROP propagation code optimized to match the existing high-energy data. This addition reduces the degeneracies in our previous analysis and when combined with an additional observed synchrotron frequency allows us to study the low-energy end of the cosmic-ray electron spectrum in a way that has not previously been done. For a pure diffusion propagation, we find a low-energy injection spectrum slightly harder than generally assumed; for J(E) \propto E^{\alpha}, we find {\alpha} = -1.34 \pm 0.12, implying a very sharp break with the spectrum above a few GeV. This then predicts a synchrotron brightness temperature spectral index, {\beta}, on the Galactic plane that is -2.8 < {\beta} < -2.74 below a few GHz and -2.98 < {\beta} < -2.91 up to 23 GHz. We find that models including cosmic-ray re-acceleration processes appear to be incompatible with the synchrotron data.