On the apparent absence of broad iron lines in Seyfert galaxies

TL;DR

This study analyzes XMM-Newton data of eleven Seyfert galaxies lacking broad iron lines, finding that relativistic reflection and ionization effects can explain the absence, challenging traditional AGN models.

Contribution

It demonstrates that relativistic and ionized reflection models can account for the missing broad iron lines in Seyfert galaxies, extending previous analyses.

Findings

Relativistic reflection models improve fits in 8/11 cases.

All 11 observations can include ionized or blurred reflection components.

Disc inclinations are generally around 60 degrees, consistent with strong relativistic effects.

Abstract

We present an analysis of XMM-Newton observations of eleven Seyfert galaxies that appear to be missing a broad iron K alpha line. These objects represent a challenge to the established paradigm for active galactic nuclei, where a relatively cold accretion disc feeds the central black hole. In that paradigm, X-ray illumination of the accretion disc should lead to continuum and fluorescence emission from iron which is broadened and shifted by relativistic effects close the hole. We extend the work of Nandra et al. (2007), who found no evidence for such a component in an earlier analysis of these objects, by testing a variety of more complex relativistic reflection models. Specifically, we consider the possibility that the disc is highly ionised, and/or that the the reflection is heavily blurred by strong relativistic effects in a Kerr geometry. We find that in 8/11 of the observations with no apparent broad iron line, the fit is significantly improved when an ionised or strongly blurred reflector is included, and that all 11 observations allow for such a component. The disc inclinations are found generally to be around 60 degrees, which when combined with a steep emissivity profile results in strong relativistic blurring of the reflection, rendering the K alpha line difficult to distinguish from the underlying continuum. Nevertheless, relativistic reflection does appear to be present, and the strength of the smeared reflection is similar to that expected from a flat disc illuminated by a point source. Such blurred reflection and the associated steep radial emissivity profiles are consistent with the gravitational light bending of the continuum photons close to the black hole.