The XMM-Newton Bright Survey sample of absorbed quasars: X-ray and accretion properties

TL;DR

This study analyzes a complete sample of X-ray absorbed quasars to determine their central engine parameters, revealing similarities with unabsorbed quasars and implications for accretion models.

Contribution

It provides the first detailed measurement of nuclear parameters for a complete sample of X-ray absorbed quasars, comparing them with unabsorbed quasars to explore their accretion properties.

Findings

Absorbed quasars have similar Eddington ratios to unabsorbed quasars.

XQSO2s occupy the 'forbidden region' in the effective Eddington limit paradigm.

No significant differences found between absorbed and unabsorbed quasar populations.

Abstract

Although absorbed quasars are extremely important for our understanding of the energetics of the Universe, the main physical parameters of their central engines are still poorly known. In this work we present and study a complete sample of 14 quasars (QSOs) that are absorbed in the X-rays (column density NH>4x10^21 cm-2 and X-ray luminosity L(2-10 keV)>10^44 ergs/s; XQSO2) belonging to the XMM-Newton Bright Serendipitous Survey (XBS). From the analysis of their ultraviolet-to-mid-infrared spectral energy distribution we can separate the nuclear emission from the host galaxy contribution, obtaining a measurement of the fundamental nuclear parameters, like the mass of the central supermassive black hole and the value of Eddington ratio, lambda_Edd. Comparing the properties of XQSO2s with those previously obtained for the X-ray unabsorbed QSOs in the XBS, we do not find any evidence that the two samples are drawn from different populations. In particular, the two samples span the same range in Eddington ratios, up to lambda_Edd=0.5; this implies that our XQSO2s populate the "forbidden region" in the so-called "effective Eddington limit paradigm". A combination of low grain abundance, presence of stars inwards of the absorber, and/or anisotropy of the disk emission, can explain this result.