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

We study the link between the X-ray emission in radio-quiet active galactic nuclei (AGN) and the accretion rate on the central supermassive black hole using a statistically well-defined and representative sample of 71 type 1 AGN 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, Gamma, and the X-ray 'loudness', and the accretion rate, both absolute ((M) over dot) and normalized to the Eddington luminosity (Eddington ratio, lambda). We parametrize the X-ray loudness using three different quantities: the bolometric correction K-bol, the two-point spectral index alpha(OX) and the disc/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 Gamma - lambda correlation. The two proxies usually adopted in the literature to quantify the X-ray loudness of an AGN, i.e. K-bol and alpha(OX), behave differently, with K-bol being more sensitive to the Eddington ratio and alpha(OX) 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.