Studying the relationship between X-ray emission and accretion in AGNs using the XMM-Newton Bright Serendipitous Survey

TL;DR

This study investigates how X-ray emission characteristics in radio-quiet AGNs relate to accretion rates, revealing dependencies of spectral slope and loudness on accretion parameters using a well-defined sample from the XMM-Newton survey.

Contribution

It provides the first detailed analysis of correlations between X-ray spectral properties and accretion rates in a statistically representative AGN sample.

Findings

X-ray spectral index correlates with normalized accretion rate.

X-ray loudness depends on both normalized and absolute accretion rates.

Different proxies for X-ray loudness show distinct sensitivities to accretion parameters.

Abstract

We study the link between the X-ray emission in radio-quiet AGNs and the accretion rate on the central Supermassive Black Hole using a statistically well-defined and representative sample of 71 type 1 AGNs extracted from the XMM-Newton Bright Serendipitous Survey. We search and quantify the statistical correlations between some fundamental parameters that characterize the X-ray emission, i.e. the X-ray spectral slope, and the X-ray "loudness", and the accretion rate, both absolute and normalized to the Eddington luminosity (Eddington ratio). We parametrize the X-ray loudness using three different quantities: the bolometric correction, the two-point spectral index and the disk/corona luminosity ratio. We find that the X-ray spectral index depends on the normalized accretion rate while the "X-ray loudness" depends on both the normalized and the absolute accretion rate. The dependence on the Eddington ratio, in particular, is probably induced by the X-ray spectral slope-Eddington ratio correlation. The two proxies usually adopted in the literature to quantify the X-ray loudness of an AGN, i.e. the bolometric correction and the two-point spectral index, behave differently, with the bolometric correction being more sensitive to the Eddington ratio and the two-point spectral index having a stronger dependence with the absolute accretion. The explanation of this result is likely related to the different sensitivity of the two parameters to the X-ray spectral index.