The Radio Background Below 100 MHz

TL;DR

This paper investigates the excess radio background below 100 MHz, supporting the existence of a strong, diffuse radio component that could explain the unexpectedly deep 21 cm absorption signal observed by EDGES.

Contribution

The study provides new observational constraints on the low-frequency radio background, supporting a model with a significant diffuse radio component beyond the cosmic microwave background.

Findings

Supports a strong, diffuse radio background with a spectral index of -2.58

Finds the background temperature at 78 MHz to be approximately 603 mK

Provides constraints on the radio background using LWA data in 40-80 MHz range

Abstract

The recent detection of the "cosmic dawn" redshifted 21 cm signal at 78 MHz by the EDGES experiment differs significantly from theoretical predictions. In particular, the absorption trough is roughly a factor of two stronger than the most optimistic theoretical models. The early interpretations of the origin of this discrepancy fall into two categories. The first is that there is increased cooling of the gas due to interactions with dark matter, while the second is that the background radiation field includes a contribution from a component in addition to the cosmic microwave background. In this paper we examine the feasibility of the second idea using new data from the first station of the Long Wavelength Array. The data span 40 to 80 MHz and provide important constraints on the present-day background in a frequency range where there are few surveys with absolute temperature calibration suitable for measuring the strength of the radio monopole. We find support for a strong, diffuse radio background that was suggested by the ARCARDE 2 results in the 3 to 10 GHz range. We find that this background is well modeled by a power law with a spectral index of $-$2.58$\pm$0.05 and a temperature at the rest frame 21 cm frequency of 603$^{+102}_{-92}$ mK.