Quasar Unification via disk winds: From Phenomenology to Physics

Quasar Unification via disk winds: From Phenomenology to Physics

Presentation by Professor Christian Knigge

On Tuesday 3 July 2018, Professor Christian Knigge (School of Physics and Astronomy – University of Southampton) gave a talk to the 4th cohort of NGCM students on modelling Quasars by unification via disk winds.

Quasars (Quasi Stellar Objects) are rapidly accreting supermassive black holes in the centres of galaxies and as the most luminous active galactic nuclei in the universe, display a bewildering array of observational properties. Their spectra, which consist of strong and broad emission lines, prove their existence. Galaxies form near quasars. However, their brightness outshines the galaxy, which means that images of them look like a point.

According to the standard QSO unification model, orientation effects are the main driver of the diversity in their observational properties. However, the physical origin of even most important observational signatures is not specified in this model and has remained largely unknown. One promising idea is that most of these signatures are formed in accretion disk winds.

In his talk, Prof. Knigge gave an overview of an ongoing collaborative research project that was designed to test the viability of QSO unification via accretion disk winds. In this scenario, most of the characteristic spectral features of QSOs are formed in these outflows. More specifically, broad absorption lines (BALs) are produced for sight lines within the outflow, while broad emission lines (BELs) are observed for other viewing angles.

In order to achieve this, they use a state-of-the-art Monte Carlo radiative transfer and photo- ionisation code to predict emergent QSO spectra for a wide range of viewing angles and quasar properties (black hole mass, accretion rate, X-ray luminosity, etc).

According to Prof. Knigge, it turns out to be relatively straightforward to produce BALs, but harder to obtain sufficiently strong BELs. They also found that it is difficult to avoid overionisation of the wind with realistic X-ray luminosities. In addition, they are using their code to test and improve hydrodynamic disk wind models for quasars. So far, they have been able to demonstrate that the treatment of ionisation in existing hydrodynamic models of line- driven disk winds is too simplistic to yield realistic results – the modelled outflows would be strongly overionised and hence would not feel the line-driving forces that are asssumed to produce them. They have therefore embarked on an effort to model line-driven disk winds self-consistently by linking a hydrodynamics code with our ionisation and radiative transfer code.

Then, Prof. Knigge stated that they can also predict the reverberation signatures produced by disk winds, which can be directly compared to the results of the latest reverberation mapping campaigns.

Finally, he described some of the results they have obtained so far and briefly outlined how they intend to achieve their ultimate goal: a fully self-consistent, multi-dimensional, multi- frequency radiation-hydrodynamic simulation of a QSO disk wind driven by radiation pressure.

Written by Dimitrios Mathas and Marley Samways