Research

My research focuses on compact objects: stellar mass and supermassive black holes, and neutron stars.

I use X-ray observations to measure the material being accreted by these objects, and the winds that they launch outwards. 

More detailed information about various aspects of my research can be found below.

MODELLING VARIANCE SPECTRA

Excess variance or RMS spectra are a quick and easy way to quantify energy-dependent variability, potentially determining the underlying cause of observed X-ray variability.

 

However, these spectra are generally under-used, because it is difficult to quantitatively model them.​

With my collaborators and students, I have begun building a library of XSPEC compatible models for fitting excess variance spectra of AGN. These models are freely available here:

ULTRA-FAST OUTFLOWS

Ultra-fast outflows are thought to be powerful winds, launched from the accretion disks of supermassive black holes and accelerated up to ~0.3c. 

These winds manifest as blueshifted X-ray absorption lines from highly-ionised material in the wind.

One of the most exciting aspects of these outflows is their potential to drive AGN feedback: driving material out of their host galaxy, leaving no fuel for star formation, and thereby regulating the growth of the galaxy.

I have pioneered the use of variability as a tool to detect and study ultra-fast outflows, including conclusively demonstrating that they respond to the illuminating X-ray continuum.

X-RAY REFLECTION

When X-rays illuminate the inner accretion disk, close to the black hole, they excite fluorescent line emission. These emission lines are then smeared and shifted by the strong relativistic effects close to the event horizon. 

This produces a characteristic "relativistic reflection" spectrum, which can be used to probe fundamental properties of the black hole, and the accretion disk surrounding it.

In my research, I develop models of X-ray reflection spectra, which can be fit to observational data and used to measure parameters such as the black hole spin and viewing angle.