UNDERSTANDING QUASARS ACROSS COSMIC TIME: THE STRUCTURE OF THE ACCRETION FLOW AROUND A SUPERMASSIVE BLACK HOLE

Black holes are the simplest possible objects, characterised only by mass and spin. We see them most easily via accretion, where the enormous gravitational potential energy released by the infalling material transforms these darkest objects in the Universe into the brightest. Hence there is another parameter which controls the observable appearance of an accreting black hole, namely the mass accretion rate, along with viewing angle for any non-spherical flow. Fundamentally, these parameters must determine the majority of the properties.

Standard disc models seem to mostly work in stellar mass black holes at high luminosities, accreting material from a binary companion star. But some very different structures are seen for similarly high luminosities in the supermassive black holes powering the Active Galactic Nuclei (AGN). A standard accretion disc in a luminous AGN with mass 108 M has peak temperature of <105K, making a very blue optical/UV continuum.

Such continuua are seen, though they are often not as blue as expected. There is a downturn in the UV emission which appears to connect to an upturn in the X-ray emission below 1 keV. The optical/UV is also variable, typically showing changes of more than 10% on timescales of weeks, yet a standard disc can only respond to changes in mass accretion rate on a timescale of > 104-years (e.g.

Noda & Done 2018). The hot X-ray corona is variable on even faster timescales (days), but again its nature is not well understood.

Instead of a standard disc, the data can be fit by models where the accretion flow structure changes. This composite geometry predicts the emission spectrum of the AGN based on mass, mass accretion rate and spin, and predicts the relative geometry of the UV and X-rays (Kubota & Done 2018).

These models are testable, as they completely specify the geometry. The variable X-rays can illuminate the soft X-ray excess and outer disc, producing a variable UV component from reprocessing which is a lagged and smoothed version of the illuminating X-ray flux (Mahmoud & Done 2020). The project will calculate this response, and use this on intensive monitoring data to explore the inner structure of AGN accretion discs, where the majority of their vast luminosity is emitted. This will give us a physical model to understand AGN across cosmic time.