Puzzling behaviour

However, in July 2022, XMM-Newton started to observe that the X-ray output was varying at levels of around 10% on timescales between 400 and 1000 seconds. Quasi-periodic oscillations (QPO), as this type of variability is called, are notoriously difficult to detect in supermassive black holes.

“This was our first indication that something strange was going on,” says Megan Masterson, PhD student at Massachusetts Institute of Technology, USA, who led the XMM-Newton study.

The oscillations could suggest that a massive object, like a star, is embedded in the accretion disk, and is rapidly orbiting the black hole on its way to being swallowed. As the object gets closer to the black hole, the time it takes to orbit decreases, causing the frequency of the oscillations to increase.

Calculations showed that this orbiting object is probably a stellar corpse known as a white dwarf, with around 0.1 times the mass of the Sun, travelling at an incredible speed. It was completing one orbit of the central monster, covering a distance of around 100 million km, every eighteen minutes or so.

Then things got even weirder.