Astrophysical radio background cannot explain the EDGES 21-cm signal: constraints from cooling of non-thermal electrons

TL;DR

The paper demonstrates that astrophysical processes cannot sustain the enhanced radio background needed to explain the EDGES 21-cm signal due to rapid cooling of relativistic electrons, challenging previous explanations.

Contribution

It provides a novel analysis showing rapid cooling timescales of electrons invalidate astrophysical explanations for the excess radio background.

Findings

Synchrotron and inverse-Compton cooling times are much shorter than the EDGES signal duration.

Cooling reduces the expected radio emission by about three orders of magnitude.

Astrophysical scenarios for the excess radio background are unlikely to explain the EDGES signal.

Abstract

Recently the EDGES experiment has claimed the detection of an absorption feature centered at 78 MHz. When interpreted as a signature of cosmic dawn, this feature appears at the correct wavelength (corresponding to a redshift range of $z\approx15-20$) but is larger by at least a factor of two in amplitude compared to the standard 21-cm models. One way to explain the excess radio absorption is by the enhancement of the diffuse radio background at $\nu = 1.42$ GHz ($\lambda=21$ cm) in the rest frame of the absorbing neutral hydrogen. Astrophysical scenarios, based on the acceleration of relativistic electrons by accretion on to supermassive black holes (SMBHs) and by supernovae (SN) from first stars, have been proposed to produce the enhanced radio background via synchrotron emission. In this Letter we show that either the synchrotron or the inverse-Compton (IC) cooling time for such electrons is at least three orders of magnitude shorter than the duration of the EDGES signal centered at $z \approx 17$, irrespective of the magnetic field strength. The synchrotron radio emission at 1.42 GHz due to rapidly cooling electrons is $\sim 10^3$ times smaller than the non-cooling estimate. Thus astrophysical scenarios for excess radio background proposed to explain the EDGES signal appear very unlikely.