My Research

I am a Royal Society University Research Fellow. I completed my PhD at the University of Durham in 2012, and was awarded the 2012 Michael Penston Thesis Prize, for the best astronomy PhD thesis in the UK that year, by the Royal Astronomical Society. I then moved to the University of Amsterdam, first as a postdoc (2012-2014) and then as an NWO Veni research fellow (2014-2017). I started my University Research Fellowship in October 2017 and was at the University of Oxford 2017-2021. I then moved to Newcastle to continue my URF and start a Lectuership in October 2021.

Most of my research has been about black hole X-ray binaries. These are binary systems consisting of a black hole about 10 times the mass of our sun and a normal star orbiting around a common centre of mass. When gas from the normal star falls onto the black hole, it spirals inwards to form an accretion disk (see the illustration to the left). Close to the black hole, the material in this disk becomes so hot that it glows extremely brightly in X-rays. So, if we point our X-ray telescopes at these objects, we get to peer into the extreme region close to the event horizon of the black hole. Here, gravity is very strong indeed, so effects predicted by General Relativity (GR) that are tiny in our own Solar System become huge! One example is the frame dragging effect. In GR, a spinning massive object twists the surrounding spacetime around with it as it rotates - you can think of this like a `gravitational vortex', or a bit like water falling down a plughole. The frame dragging effect means that the motion of satellites in orbit around the Earth is affected just because the Earth is spinning. Specifically, the satellite's orbit will wobble vertically. This is called Lense-Thirring precession after the authors who fist predicted it shortly after Einstein published his theory of GR. Since the Earth's gravitational field is pretty weak, the wobble is extremely slow. Extremely accurate measurements were therefore needed for NASA's Gravity Probe B to finally detect it in 2011. Close to a stellar-mass black hole though, the wobble is about 10 000 000 000 000 000 faster! So around a black hole, the plane of the orbit should wobble from one side to the other and back again in about a second or even less, whereas around the Earth this would instead take 33 million years! Fascinatingly, we routinely see nearly periodic flashing in the X-ray brightness of X-ray binaries, with a period of about a second. These are called quasi-periodic oscillations (QPOs). They were first discovered by Michiel van der Klis in the 1980s and it was first suggested that they might be something to do with Lense-Thirring precession by Stella & Vietri in 1998. However, Stella & Vietri's model just considered precession of test particles - i.e. they calculated the precession period that a satellite orbiting the black hole would have. We unfortunately can't put a satellite in orbit around a black hole, we can only make use of the natural experiments that we witness when gas falls onto a black hole in an X-ray binary, and in this case we have an accretion disk instead of test particles.

During my PhD, I suggested that QPOs happen because the entire inner part of the accretion disk wobbles due to the frame dragging effect. This way, as we see the inner disk from different angles, it appears to have a different brightness, meaning that we see a QPO. This was inspired by a numerical simulation ran by Chris Fragile the previous year. He had set up a simulation with gas falling onto a spinning black hole from a direction misaligned with the spin plane, including all GR effects. In this simulation, the entire disk precessed together because of the frame dragging effect (see Chris' website for cool movies of this simulation). This was for a small disk, and I hypothesised that in a real situation where we have a very large disk, just the inner part will precess. This is illustrated by the animation to the right: the orange inner disk precesses and the outer grey disk remains stationary. I showed that this model reproduces a lot of the observed properties of QPOs. However, the frame dragging effect alone would just cause a periodic oscillation, not a QPO. So later on in my PhD, I suggested that fluctuations in the density of the precessing region will make the precession quasi-periodic; i.e. the wobble is noisy rather than predictable. These fluctuations in the density are needed to explain the non-periodic flashing that we also see in the X-ray signal at the same time as QPOs. I developed a model that could explain both the non-periodic flashing and the QPO, and compared this with the observational data in order to try and measure properties of the accretion flow. This was the first time this had been done at the time.

I 2012, I moved to the Anton Pannekoek Institute at the University of Amsterdam to work as a postdoc of Michiel van der Klis. In Amsterdam, I continued to work on my model for the non-periodic flashing. The problem with the modelling I'd done during my PhD was that, to model the fluctuations in disk density, I had to run simulations whereby I set up randomised fluctuations in the disk material and followed them as they fell towards the black hole. This was extremely time consuming, and so made it hard to compare the model with real observations. The model is mathematically well defined though, and so we worked out a mathematical formula that did exactly the same thing as the simulation. This made my model run about 1000 times faster! With the model running a lot faster, it was now possible to compare it to data for lots of observations, and therefore measure properties of black hole accretion disks. This was the PhD project of Stefano Rapisarda, who I co-supervised.

In 2014, I was awarded the Veni fellowship by NWO to continue my work in Amsterdam. The plan was to test a distinctive prediction of the precession model that I'd made a few years earlier. This uses the X-rays from the inner disk that reflect off the outer disk and scatter back into our line of sight. We can isolate these reflected X-rays because a lot of them have an energy of about 6.4 keV. This is due to fluorescence of iron atoms in the disk. We can think of this like neon signs that glow bright red: this happens because neon atoms fluoresce red light. Iron atoms instead fluoresce X-ray light! The excess X-ray emission at about 6.4 keV is called an iron fluorescence line, and it is very important because it would be a narrow spectral line if the material in the disk were not moving and if the gravitational field of the black hole were not so strong. But, material in the disk is orbiting the black hole very rapidly. This means that iron emission from the approaching disk material is Doppler blue-shifted and iron emission from the receding side of the disk is Doppler red-shifted. Also, X-rays lose energy when they escape the gravitational pull of the black hole -- this is called gravitational redshift. The closer to the black hole the X-rays were emitted from, the redder they appear to be when they finally reach our telescope. So the iron line that we see is broadened by all of these effects. We can therefore try to measure properties of the black hole and its accretion disk by modelling exactly how this line is broadened. What I had pointed out was that, if the inner disk is actually wobbling, then the iron line will look different to us when the inner disk is tilted in different directions. When it shines on the approaching side of the disk ('a' in the illustration on the left), we will see a very blue iron line. When it shines on the receding side of the disk ('c' in the illustration on the left), we will see a very red iron line. My plan was to look for this predicted wobbling of the line energy. In order to do this, I invented a new observational technique and arranged a very long observation of a QPO in an X-ray binary called H 1743-322 using the European Space Agency's XMM-Newton and NASA's NuSTAR. When I analysed the data, it did show the predicted wobbling of the iron line energy! This is the best evidence we have that QPOs are indeed the result of Lense-Thirring precession.

Since October 2017, I've been a Royal Society University Research Fellow at the University of Oxford. I still co-supervise two PhD students from the University of Amsterdam. One student, Matthew Liska, works on numerical simulations -- the kind that Chris Fragile first ran in 2007. 10 years later though, computers are a lot faster, so we can run simulations with a much higher resolution to capture more details of the physics. On top of this, Matthew has developed the fastest, most efficient code of it's kind in the world. The code is called H-AMR -- pronounced `hammer'! With this code, we have achieved a number of breakthroughs. As Chris Fragile saw in 2007, we also find that the entire accretion disk wobbles due to the frame dragging effect, but in our simulation we also see that a narrow jet of material is launched away from the black hole. We see these jets all the time around black holes, and they are very important because they allow black holes to heat up material around them. Understanding how these jets work is therefore the subject of a lot of theoretical effort. We found in our simulation that the jet precesses as well as the disk. We have also been able to study disks that are thinner than those studied before, which is important because we think that real disks are very thin. My other Amsterdam student, Guglielmo Mastroserio, is working on reverberation mapping of accretion black holes. This technique measures the small time delay between the X-rays from the inner disk that we see directly, and those that reflected from the disk. Because the X-rays that reflected from the disk take an ever-so-slightly longer path to reach us than the X-rays that travel directly towards us, the iron line emission is very slightly delayed compared with the rest of the X-ray emission. We can use this to measure things about the disk and the black hole. We have recently developed this technique into a new way to measure the mass of the black hole. This is possible because the broadening of the iron line depends on the size of the accretion disk divided by the black hole mass. The delay though just depends on the size of the disk. So if we measure the absolute size of the disk using the delay time, we can get a measure of the black hole mass from the broadening of the iron line! I now have a new PhD student, Ed Nathan. He is going to be applying my QPO and reverberation techniques to data from NASA's new X-ray mission NICER (the Neutron star Interior Composition ExpolreR) in order to measure properties of black holes and their accretion disks. NICER can take very high quality data, allowing us to explore the physics in greater detail than ever before.

In the future, I'm very excited about X-ray polarimetry. In order to understand polarization, we need to appreciate that light is a wave-like oscillation of an electric field. For a vertically polarized light ray, the oscillation is vertical. For a horizontally polarized light ray, the oscillation is horizontal. From any light source, we see lots of light rays. If the source is unpolarized, we see just as many vertically polarized light rays as we see horizontally polarized light rays. For a source to be vertically polarized, we see more vertically polarized light rays than horizontally polarized light rays. Polarization is a property of light that is very difficult to measure for X-rays, but is fairly easy to measure for other wavelengths. For example, 3D glasses work using polarization: the cinema screen projects two slightly different images, one a little to the left of the other (try peaking at the cinema screen without your glasses during a 3D film). If the lefthand image is 100% horizontally polrized and the righthand is 100% vertically polarized, a 3D effect can be created if the lefthand lense of the glasses only lets in horizontally polarized light, blocking vertically polarized light, and the righthand lense of the glasses only lets in vertically polarized light. This way, your left eye sees something slightly different from your left eye -- which is exactly what happens when you look at a 3D object, since each eye looks from a slightly different angle! So polarization of optical light is measured all the time, but the last time we had an X-ray polarimeter in space was in the late 1970s! However, NASA are due to launch a new X-ray polarimeter in 2021, called IXPE (the Imaging X-ray Polarimetry Explorer). This will allow us to make completely new kinds of observations of black holes. For example, if the inner disk really is precessing, the X-ray polarization should rock back and forth as the inner disk wobbles. I am already developing the observational techniques required to measure this rocking back and forth of X-ray polarization.