The results, obtained with ESA’s XMM-Newton space X-ray telescope, paint a fascinating picture that defies how we thought matter falls into such black holes, and points to a potential source of gravitational waves that ESA’s future mission, LISA, could see.

The study, led by Megan Masterson, PhD student at Massachusetts Institute of Technology, USA, shows us that black holes devour matter in more complex ways than astronomers first thought. Black holes are predictions of Albert Einstein’s general relativity. They are gravitational monsters that imprison any piece of matter or energy that crosses its ‘surface’, a region of spacetime known as the event horizon.                                                                       

During its final descent into the black hole, a process known as accretion, the doomed matter forms a disc around the black hole. The gas in the accretion disc heats up and gives off mostly ultraviolet (UV) light.

The UV rays interact with a cloud of electrically charged gas, or plasma, that surrounds the black hole and accretion disc. This cloud is known as the corona and the interactions give the UVs a boost up to X-ray energies, which XMM-Newton can capture.

XMM-Newton has been observing the supermassive black hole (SMBH) 1ES 1927+654 since 2011. Back then, everything was pretty normal but in 2018, things changed.

1ES 1927+654 suffered a large outburst that appeared to disrupt its surroundings because the X-ray corona disappeared. Gradually, the corona returned and by early 2021, normality appeared to have been restored.

However, in July 2022, XMM-Newton started to observe that the X-ray output was varying almost periodically, at levels of around 10% on timescales between 400 and 1000 seconds. Such nearly regular variations in the brightness are known as quasi-periodic oscillations (QPOs).

"Something more exotic playing out"

“We routinely see QPOs from smaller black holes in our own Galaxy,” says Dr Adam Ingram, Senior Lecturer at Newcastle University's School of Mathematics, Statistics and Physics, who is a co-author of the study, “but they are notoriously difficult to detect from SMBHs. The QPOs we see from 1ES 1927+654 are remarkably clear though, and they look in some ways like the QPOs we see from small black holes in our Galaxy, but ultimately the properties don’t quite match up and so we must be seeing something more exotic playing out”.

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 the black hole 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 SMBH, a distance of around 100 million kilometres, every eighteen minutes or so.

Then things got even weirder

Over the course of almost two years, XMM-Newton showed that the oscillations were increasing in strength and frequency – but not in the way the researchers expected.

The team assumed that the object’s orbital energy was being emitted as gravitational waves as dictated by General Relativity. To test the idea, Megan calculated when this object would cross the event horizon, disappear from view, and the oscillations would stop. It turned out to be 4 January 2024.

“I've never in my career been able to make a prediction that precisely before,” says Erin Kara, Massachusetts Institute of Technology, and Megan’s PhD supervisor.

In March, XMM-Newton looked again – and the oscillations were still there. The object was now travelling at about half the speed of light and completing an orbit every seven minutes. Whatever was in the accretion disc, it was stubbornly refusing to be devoured by the black hole. Either there were more than just gravitational waves at play or the entire hypothesis needed to change.

The researchers also considered another possibility for the origin of the oscillations. Remembering the disappearance of the X-ray corona in 2018, they wondered whether this cloud itself could be oscillating.

The trouble was that there is no established theory to explain such behaviour and so with no clear path to further this idea, they returned to the original model, and realised that there was a way to modify it.

Astronomers have discovered pairs of white dwarfs gradually spiralling towards each other. As they close in, instead of remaining intact, one can start pulling matter off the other, and this slows down the approach of the two objects. Could the same be happening here and instead of heading intact into the black hole, the white dwarf is being slowly ripped apart?

There is a way to decide

In the 2030s, ESA will launch the Laser Interferometer Space Antenna (LISA), which is designed to detect gravitational waves in exactly the frequency range that 1ES 1927+654 is emitting. “We predict that if there is a white dwarf in orbit around this supermassive black hole, LISA should see it,” says Megan. If so, it would be a fascinating look at what happens close to a SMBH.

Reference:

Masterson, M., Kara, E., Panagiotou, C., Alston, W. N., Chakraborty, J., Burdge, K., ... & Wang, J. (2025). Millihertz Oscillations Near the Innermost Orbit of a Supermassive Black Hole. arXiv preprint arXiv:2501.01581. DOI: 10.48550/arxiv.2501.01581