The Timing Revolution of Accreting Black Holes

Black holes are the most extreme objects we know of in the universe. They are places where our understanding of physics breaks down. A one-way passage to the unknown.

Despite their extreme nature, black holes can be described by two simple properties: their mass and spin. Mass sets the radius of the event horizon, the last boundary before light can escape. Spin is a measure of the rotation of space around the event horizon.

Measuring these two simple properties is at the heart of many outstanding challenges in astrophysics.

Astronomers now know that a supermassive black hole lies in the centre of every galaxy we can observe throughout the universe. The size and mass of these black holes is proportional to the size and mass of the galaxy in which it resides. This tells us the galaxy and black hole formed together in the early universe, and have coevolved together. Understanding the history of the universe means understanding the birth, growth and evolution of the black holes within these galaxies.

Black holes grow by accreting gas from their surroundings, which we know as active galaxies. This accretion process is not static; as the amount of material falling into the black hole changes over time, so does the amount of electromagnetic radiation (light) produced as the gas heats up before it plunges into the hole. When we point our telescopes at 'these active galaxies', we see the brightness vary over time, in a similar way to stock market prices varying over the course of a day or year.

We can use similar mathematical tools to economists to try and decode what is causing this intense volatility.

One such method is known as X-ray reverberation mapping. We're familiar with the concept of reverberation; everyone's a fantastic singer in the shower. The reason for this is the size, geometry and material of a room change the way we hear ourselves.

When we hear someone talk in a classroom or a cathedral, our ears can easily detect the difference and identify the room. In a similar manner, we use X-ray reverberation to decode the size and geometry of the gas spirally around the black hole. How close the gas gets to the event horizon and how fast it is rotating is set by the mass and spin of the black hole.

If we can spatially map this gas then we can determine these two fundamental properties. In this research program, we will develop cutting-edge reverberation mapping models and apply to all available X-ray data - turning our static picture into a movie! This modelling will also reveal properties of the high velocity winds and jets expelled in the process.

This feedback of gas provides the link between the central black hole and host galaxy as they evolve.

Some black holes display a periodic change in brightness, much like daily trends we experience in temperature variation. The project lead has recently acquired a month-long dataset on a black hole with a mass a million times that of the sun, with a periodicity of an hour. This tells us whatever is causing the variation is occurring on the last stable orbit before gas plunges into the black hole.

We will identify the mechanism producing the periodic variation enabling us to perform new tests on the nature of spacetime around a black hole.

As well as deep observations on individual sources, astronomers have been systematically scanning the sky for several decades and collecting brightness measurements on millions of active galaxies. These 'big data' surveys allow us to systematically study the properties of active galaxies, deep into the distant and therefore early universe. We will develop and implement novel machine learning algorithms in order to us the source brightness variations to interesting properties, such as whether there are in fact two hidden supermassive black holes orbiting each other.

This cutting-edge work will have an impact on many areas of astrophysics, physics and cosmology, as well as paving the way for observatories of the future.