X-ray timing studies of black holes

In X-ray timing studies of black holes, we study and characterize the variability in the X-ray emission from gas spiralling into black holes. Often, such studies are performed in black hole X-ray binary systems: systems where a normal star orbits around a stellar-mass black hole and slowly loses mass towards this black hole. In such binary systems, my work focuses on quasi-periodic oscillations, or QPOs, which are variable signals in the X-ray brighntess that are almost, but not perfectly, periodic. As these signal originate from the direct vicinity of the black hole, they allow us to probe the effects of extreme gravity on matter and space. However, such timing analyses can also be performed on supermassive black holes in the centres of galaxies, as shown in the fourth project listed on this page, regarding Sagittarius A*.

Inclination dependence of QPOs

In 2017, my collaborators and I examined all Rossi X-ray Timing Explorer observations of fifteen accreting black holes in our Galaxy. We discovered that the lag of the QPO - whether the signal arrives first at high or low X-ray energies - depends on our viewing angle of the binary. This simple but striking effect presents strong evidence that the QPO is caused by a geometric process, such as General Relativistic Lense-Thirring precession close to the black hole.

Differential precession

The famous accreting black hole GRS 1915+105 shows odd behaviour in many ways; one is that its QPO frequency displays a dependence on energy. As this does not fit with canonical QPO models, we set out to test whether this may be a selection effect, due to a correlation between hardness of the emission and QPO frequency, or could be an intrinsic QPO property. Surprisingly, we find that the latter is the case. This lead us to argue that differential precession, which was not seen before, may explain this variability pattern in GRS 1915+105.

Do QPOs have waveforms?

Many advanced studies of QPO try to find changes in the X-ray spectrum, as a function of phase in the QPO signal. However, as QPOs are not perfectly periodic, subsequent cycles of the signal do not necessarily have the same waveform. In this work, lead by Iris de Ruiter (now PhD student at the University of Amsterdam), we explicitly tested whether accreting black holes show consistent waveforms. We found that this is indeed the case, which supports the idea that we can systematically search for changes within a QPO cycle by averaging cycles together.

Does the flaring of Sagittarius A* change over time?

As all galaxies do, the Milky Way hosts a supermassive black hole in its centre, with an estimated mass of four million Solar masses. Due to its proximity, we know that Sgr A* shows flares in its infra-red and X-ray emission on a, roughly, daily basis. However, the precise origin of these flares remains poorly understood. In this project, student-lead by Alexis Andres from El Salvador, we studied a unique dataset: observations with the space-based Swift X-ray observatory that began in 2006 and have since repeated at least several times a week, whenever Sgr A* was far enough from the Sun. We find evidence for a change in the brightness distribution of the X-ray flares on time scales of year, apparently decreasing in brightness between 2008 and 2012. Alexis started this project as part of ASPIRE, a wonderful project to allow talented students from backgrounds with less opportunities for astronomical research to travel to Amsterdam and do a summer project.