Can the Local Bubble explain the radio background?

TL;DR

This paper investigates whether the Local Bubble's magnetic field and cosmic ray electrons can explain the unexplained radio background detected below 3 GHz, using a turbulent magnetic field model and observational constraints.

Contribution

The study models the Local Bubble's radio emission considering cosmic rays and magnetic fields, providing constraints on its contribution to the radio background.

Findings

Model with 3-5 nT magnetic field matches observed temperatures

Lower magnetic fields (~0.2-0.6 nT) imply minimal contribution to the background

Predicted spectrum curvature suggests other sources below 100 MHz

Abstract

The ARCADE 2 balloon bolometer along with a number of other instruments have detected what appears to be a radio synchrotron background at frequencies below about 3 GHz. Neither extragalactic radio sources nor diffuse Galactic emission can currently account for this finding. We use the locally measured Cosmic ray electron population, demodulated for effects of the Solar wind, and other observational constraints combined with a turbulent magnetic field model to predict the radio synchrotron emission for the Local Bubble. We find that the spectral index of the modelled radio emission is roughly consistent with the radio background. Our model can approximately reproduce the observed antenna temperatures for a mean magnetic field strength B between 3-5 nT. We argue that this would not violate observational constraints from pulsar measurements. However, the curvature in the predicted spectrum would mean that other, so far unknown sources would have to contribute below 100 MHz. Also, the magnetic energy density would then dominate over thermal and cosmic ray electron energy density, likely causing an inverse magnetic cascade with large variations of the radio emission in different sky directions as well as high polarisation. We argue that this disagrees with several observations and thus that the magnetic field is probably much lower, quite possibly limited by equipartition with the energy density in relativistic or thermal particles (B = 0.2-0.6 nT). In the latter case, we predict a contribution of the Local Bubble to the unexplained radio background at most at the per cent level.