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Exploring the origins and implications of variability in X-ray binary systems and active galactic nuclei

Final Activity Report Summary - XRB/AGN VARIABILITY (Exploring the origins and implications of variability in X-ray binary systems and active galactic nuclei)

The project was in the area of astrophysics and astronomy, with the aim to study and understand the fluctuations seen in the X-ray emission of black holes in our galaxy. The X-rays are thought to come from close to the black holes, where gravitational energy is released from gas as it falls in towards the black hole.

The reason why the X-ray emission varies has long been a mystery, but it could hold vital clues to the structure of the flow of gas on to the black hole, and the nature of the processes that produce the X-ray emission. If we can understand these processes, we can make great progress to understanding the physics of matter in conditions of extreme gravity, and gain clues as to how black hole energise their environments, e.g. through the mysterious 'state transitions' which lead to the formation of powerful jets of material away from the black hole.

The key result of this project is that X-ray fluctuations were observed coming directly from the accretion disk, gas formed in to a disk as it spirals in to the black hole. This shows that the fluctuations are formed directly in the flow of gas, perhaps due to turbulence. We also observed that the fluctuations got faster in time as the X-ray emission got slightly brighter. This result can be explained if the disk gets thicker over time, since gas moves faster through the disk when it is thicker.

This possibility is highly interesting for interpreting the variability, since a change in flow thickness had never before been considered as an explanation for changes in the timing properties of black holes, so the result has implications for understanding variations in many more black hole systems.