XMM-Newton shows us that black holes devour matter in more complex ways than astronomers thought. Black holes are predictions of Albert Einstein's theory of general relativity. They are gravitational monsters that trap any piece of matter or energy that crosses their “surface”, a region of space-time 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 emits mainly 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 a corona, and the interactions give energy to UV rays, propelling them to X-rays, which XMM-Newton can capture.
XMM-Newton has been observing the supermassive black hole 1ES 1927+654 since 2011. Back then, everything was pretty much normal. But in 2018, things changed.
1ES 1927+654 experienced a large eruption that appeared to disrupt its surroundings, as the X-ray corona disappeared. Gradually the corona returned, and by the start of 2021 normality seemed to have been restored.
A black hole with confusing behavior
However, in July 2022, XMM-Newton began observing that X-ray production varied at levels of around 10% on time scales between 400 and 1,000 seconds. Quasi-periodic oscillations (QPOs), as this type of variability is called, are notoriously difficult to detect in supermassive black holes.
“It was our first indication that something strange was going on“, explains Megan Masterson, doctoral student at Massachusetts Institute of Technology (United States), who led the XMM-Newton study.
The oscillations could suggest that a massive object, like a star, was embedded in the accretion disk and was spinning rapidly around the black hole before being swallowed. As the object got closer to the black hole, the time it took to orbit decreased, leading to an increase in the frequency of the oscillations.
Calculations showed that this orbiting object was likely a stellar corpse known as a white dwarf, which has a mass of about 0.1 times that of the Sun and moves at incredible speeds. The ex-star was orbiting the central monster at breakneck speed, covering a distance of about 100 million km, every eighteen minutes or so.
Then things got even stranger.
For nearly two years, XMM-Newton showed that the oscillations increased in strength and frequency, but not in the way the researchers expected.
The team assumed that the object's orbital energy was emitted as gravitational waves, consistent with general relativity. To test this hypothesis, Masterson calculated when the object would cross the event horizon, disappear from view, and the oscillations would cease. It turned out to be January 4, 2024.
“In my career, I have never been able to make such an accurate prediction” says Erin Kara, of the Massachusetts Institute of Technology and Megan's thesis advisor.
XMM-Newton continued the investigation
In March 2024, XMM-Newton looked again and the oscillations were still there. The object then moved at about half the speed of light and completed one orbit every seven minutes. Whatever the contents of the accretion disk, it stubbornly refused to be devoured by the black hole. Either there was more than gravitational waves at play, or the whole hypothesis needed to be changed.
The researchers also considered another possibility for the origin of the oscillations. Remembering the disappearance of the X-ray corona in 2018, they wondered if this cloud might itself be oscillating.
The problem is that there is no established theory to explain this behavior and, having no clear path to take this idea forward, they went back to the original model and realized that there was a way to modify it.
Astronomers have discovered pairs of white dwarfs gradually spiraling closer to each other. As they get closer, instead of remaining intact, one of them may begin to tear material from the other, slowing down the approach of both objects.
Could it be that the same is happening here and that instead of heading towards the black hole intact, the white dwarf is slowly being torn apart?
There is a way to find out.
LISA's fresh perspective on this matter
In the 2030s, ESA will launch the Laser Interferometer Space Antenna, or LISA, designed to detect gravitational waves exactly in the frequency range emitted by 1ES 1927+654. “We predict that if there is a white dwarf orbiting this supermassive black hole, LISA should see it“, says Megan Masterson, author of a study on this surprising discovery. If true, it would be a fascinating look at what is happening so close to the gravitational monster.
“This is another great example of XMM-Newton’s unique capabilities. It played a critical role in this result and is the only observatory capable of capturing this QPO signal with such clarity. The detection relied on XMM-Newton's exquisite combination of long observations, large collection area across the entire X-ray band, and temporal resolution“, said Norbert Schartel, ESA's XMM-Newton project scientist.
Source: ESA / NASA
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