On the radio -- X-ray luminosity correlation of radio halos at low radio frequency - Application of the turbulent re-acceleration model

TL;DR

This paper predicts that low-frequency radio observations will reveal a new population of ultra-steep spectrum radio halos, affecting the radio-X-ray luminosity correlation and providing a test for the turbulent re-acceleration model.

Contribution

It introduces expectations for low-frequency radio halos and demonstrates how upcoming LOFAR surveys can test the turbulent re-acceleration scenario.

Findings

Ultra-steep spectrum halos cause a steepening of the correlation at 120 MHz.

The correlation broadens at low frequencies compared to 1.4 GHz.

LOFAR surveys will be capable of detecting these halos and testing the model.

Abstract

In this paper we show expectations on the radio--X-ray luminosity correlation of radio halos at 120 MHz. According to the "turbulent re-acceleration scenario", low frequency observations are expected to detect a new population of radio halos that, due to their ultra-steep spectra, are missed by present observations at ~ GHz frequencies. These radio halos should also be less luminous than presently observed halos hosted in clusters with the same X-ray luminosity. Making use of Monte Carlo procedures, we show that the presence of these ultra-steep spectrum halos at 120 MHz causes a steepening and a broadening of the correlation between the synchrotron power and the cluster X-ray luminosity with respect to that observed at 1.4 GHz. We investigate the role of future low frequency radio surveys, and find that the upcoming LOFAR surveys will be able to test these expectations.