XMM-Newton observations of 4 luminous radio-quiet AGN, and the soft X-ray excess problem

TL;DR

This study analyzes XMM-Newton data of four luminous radio-quiet AGN to investigate the soft X-ray excess, testing various models, and finds no fully satisfactory explanation, highlighting the need for broader energy observations.

Contribution

The paper compares multiple models for the soft X-ray excess in AGN using observational data, revealing similar parameters across sources and the limitations of current models.

Findings

Comptonization and smeared wind models fit data well

Parameters are similar across sources despite different properties

No definitive model explains the soft X-ray excess

Abstract

The nature and origin of the soft X-ray excess in radio quiet AGN is still an open issue. The interpretation in terms of thermal disc emission has been challanged by the discovery of the constancy of the effective temperature despite the wide range of Black Hole masses of the observed sources. Alternative models are reflection from ionized matter and absorption in a relativistically smeared wind. We analyzed XMM-Newton observations of four luminous radio quiet AGN with the aim of characterising their main properties and in particular the soft excess. Different spectral models for the soft excess were tried: thermal disc emission, Comptonization, ionized reflection, relativistically smeared winds. Comptonization of thermal emission and the smeared winds provide the best fits, but the other models also provide acceptable fits. All models, however, return parameters very similar from source to source, despite the large differences in luminosities, Black Hole masses and Eddington ratios. Moreover, the smeared wind model require very large smearing velocities. The UV to X-ray fluxes ratios are very different, but do not correlate with any other parameter. No fully satisfactory explanation for the soft X-ray excess is found. Better data, like e.g. observations in a broader energy band, are needed to make further progresses.