In 2018, astronomers observed that the corona of 1ES 1927+654, an actively accreting, 1.4-million-solar-mass black hole located in a galaxy around 270 million light-years away, suddenly disappeared, before reassembling months later. The brief though dramatic shut-off was a first in black hole astronomy. Now, astronomers using ESA’s XMM-Newton observatory caught the same black hole exhibiting more unprecedented behavior. They detected flashes of X-rays coming from 1ES 1927+654 at a steadily increasing clip; over a period of two years, the flashes, at millihertz oscillations, increased in frequency from every 18 minutes to every seven minutes; this dramatic speed-up in X-rays has not been seen from a black hole until now.
In this artist’s concept, matter is stripped from a white dwarf (sphere at lower right) orbiting within the innermost accretion disk surrounding 1ES 1927+654’s supermassive black hole. Image credit: NASA / Aurore Simonnet, Sonoma State University.
Black holes are predictions of Albert Einstein’s theory of general relativity. They are gravitational monsters that imprison any piece of matter or energy that crosses their ‘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 disk around the black hole. The gas in the accretion disk 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 disk. This cloud is known as the corona and the interactions give the UV rays energy, boosting them up to X-rays, which XMM-Newton can capture.
XMM-Newton has been observing 1ES 1927+654 since 2011. Back then, everything was pretty normal.
But in 2018, things changed — the black hole 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 at levels of around 10% on timescales between 400 and 1,000 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,” said Megan Masterson, a Ph.D. student at MIT.
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, traveling 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.
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.
They assumed that the object’s orbital energy was being emitted as gravitational waves as dictated by general relativity.
To test the idea, they calculated when this object would cross the event horizon, disappear from view, and the oscillations would stop — it turned out to be January 4, 2024.
“I’ve never in my career been able to make a prediction that precisely before,” said MIT’s Dr. Erin Kara.
In March 2024, XMM-Newton looked again — and the oscillations were still there.
The object was now traveling at about half the speed of light and completing an orbit every seven minutes.
Whatever was in the accretion disk, 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 astronomers 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 behavior and so with no clear path to further this idea, they returned to the original model, and realized that there was a way to modify it.
“If the black hole does have a white dwarf companion, the gravitational waves it produces will be detectable by LISA, an ESA mission in partnership with NASA that is expected to launch in the next decade,” Masterson said.
The team’s paper will appear in the journal Nature.
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Megan Masterson et al. 2025. Millihertz Oscillations near the Innermost Orbit of a Supermassive Black Hole. Nature, in press; arXiv: 2501.01581
