The UV peak in Active Galactic Nuclei : a false continuum from blurred reflection ?

TL;DR

This paper proposes that the UV bump in Active Galactic Nuclei is a false continuum caused by blurred reflection from cold, thick clouds near the accretion disc, resolving multiple observational problems.

Contribution

It introduces a model involving cold, dense clouds at ~30R_S that reprocess the continuum, explaining the UV bump and related spectral features in AGN.

Findings

Cloud reflection can produce a false UV continuum peak.

The model fits observed SEDs and explains UV variability.

Small mass of clouds suffices without disrupting the disc.

Abstract

I summarise and analyse key problems with observations of the UV bump in Active Galactic Nuclei (AGN), and especially the accretion disc interpretation - the temperature problem, the ionisation problem, the timescale problem, and the co-ordination problem - and suggest that all these problems can be solved if, in addition to the accretion disc, there is a population of cold, thick clouds at approximately 30R_S which reprocess the intrinsic continuum. Exploring cloud parameter space, I find that clouds with density n~10^12 cm^-3 and column N_H >4 X 10^24 cm^-2 reflect most of the intrinsic continuum, but convert a substantial fraction of the EUV luminosity into lines, dominated by Ly-beta and HeII Ly-alpha. When velocity-blurred, this makes a false continuum peak at ~1100A which fits the observed SED well, but turns back up in the FUV to make a hard EUV SED, as required by ionisation models. I argue that the observed UV variability is dominated by this component of fixed shape, possibly due to changes of covering factor. The amount of mass required is small, so it is not necessary to disrupt the disc, but only to make an unstable and inhomogeneous atmosphere. The proposed clouds may be related to those suggested by several X-ray phenomena (X-ray reflection components, high-velocity outflows, Compton thick partial covering) but are not the same, leading to a picture with a wide range of inhomogeneous structures at different radii.