The AT20G view of Swift/BAT selected AGN: high-frequency radio waves meet hard X-rays

TL;DR

This study cross-matched high-frequency radio and hard X-ray surveys of AGN, finding no correlation between radio and X-ray luminosities at high frequencies, suggesting the radio-X connection weakens at these energies.

Contribution

It provides the first high-frequency radio and hard X-ray correlation analysis for AGN, revealing the disappearance of the radio-X connection at these energies.

Findings

Approximately 20% of hard X-ray detected AGN are bright at 20 GHz.

No correlation between 20 GHz radio and 15-55 keV X-ray luminosities in local Seyfert AGN.

The radio-X connection observed at lower frequencies does not persist at high frequencies.

Abstract

We cross-matched the 6-year Swift/Burst Alert Telescope (BAT) survey of active galactic nuclei (AGN) with the AT20G radio survey of the southern sky, which is one of the largest high-frequency radio surveys available. With these data we investigated the possible correlation between the radio and the X-ray emission at the highest radio and X-ray frequencies. We found 37 AGN with a high probability of association (>80 per cent), among which 19 are local Seyfert galaxies (with median redshift z = 0.03) and 18 blazars. We found that \approx 20 per cent of the AGN detected in hard X-rays are also bright radio sources at 20 GHz, but the apparent correlation between the radio and hard X-ray luminosity is completely driven by the different median redshifts of the two subgroups of AGN. When we consider only the local Seyfert sample we find no evidence of a correlation between their 20 GHz and 15-55 keV power. Therefore it appears that at high frequencies the radio-X connection, which had been previously observed at lower frequencies, disappears. The disappearance of the radio-X correlation at high radio and X-ray frequencies could be tested through Very Long Baseline Interferometry and the use of the Nuclear Spectroscopic Telescope Array (NuSTAR) satellite.