Comparison of hard X-ray spectra of luminous radio galaxies and their radio-quiet counterparts

TL;DR

This study compares the X-ray properties of luminous radio galaxies and their radio-quiet counterparts, revealing similar spectral slopes but higher X-ray luminosities in radio-loud AGNs, suggesting common emission mechanisms and differences in accretion efficiency.

Contribution

It provides a controlled comparison of X-ray spectra between radio-loud and radio-quiet AGNs, highlighting similarities in spectral slopes and differences in luminosity linked to accretion efficiency.

Findings

Radio galaxies are X-ray louder than radio-quiet AGNs.

Spectral slopes are very similar in both types.

Higher X-ray luminosities in radio-loud AGNs may be due to faster black hole spins.

Abstract

We study the differences in X-ray properties of luminous radio galaxies and their radio-quiet counterparts. In order to avoid biases associated with the dependence of X-ray properties on the black hole mass and Eddington ratio, the radio-loud and radio-quiet objects are selected to cover similar ranges of these parameters. Our studies are based on the X-ray data from the Swift/BAT catalogue. We confirm previous results that radio galaxies are on average X-ray-louder than radio-quiet AGNs, but find that their spectral slopes are very similar. This suggests that in radio-loud and radio-quiet AGNs the hard X-rays are produced in the same region and by the same mechanism. We argue that this region can be associated with the hot, geometrically thick, central portion of the accretion flow, where production of hard X-rays is likely to be dominated by Comptonization of the optical/UV radiation of the truncated 'cold' accretion disc by hot electrons. The larger X-ray luminosities in radio-loud AGNs may result from larger radiative efficiencies of the innermost portions of the accretion flows around faster rotating black holes.