XMM-Newton observes black hole that exhibits strange oscillations

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The European Space Agency’s (ESA) XMM-Newton X-ray space telescope recently discovered rapidly fluctuating X-ray signals coming from the edge of supermassive black hole 1ES 1927+654. Located at the center of a nearby galaxy, the black hole’s X-ray signals have given scientists insight into its nature and how matter falls into it.

Moreover, XMM-Newton’s observations also point to a new potential source for gravitational waves. Gravitational waves are most commonly produced by binary black hole systems in which two black holes orbit one another. However, the gravitational waves produced by the black hole in XMM-Newton’s observations result from a mysterious object orbiting the black hole.

Before matter ultimately falls into a black hole, it must first orbit the outer edge of the black hole — a region of spacetime called the “event horizon.” As matter orbits outside the event horizon, it gradually heats up, creating a disk of glowing material around the black hole called an “accretion disk.” The gas and matter within an accretion disk often heat up so much that the disk emits primarily ultraviolet (UV) light.

The accretion disk’s UV rays then interact with the black hole’s corona, the plasma cloud surrounding the entire black hole and accretion disk. The corona’s high energy often turns the accretion disk’s UV rays into higher-energy X-rays, which XMM-Newton can observe.

Artist’s illustration of XMM-Newton. (Credit: ESA/C. Carreau)

Since 2011, XMM-Newton has regularly observed 1ES 1927+654, noting the corona and X-rays originating from the black hole’s accretion disk. However, in 2018, scientists noticed a significant change in 1ES 1927+654, most notably that its X-ray corona had seemingly vanished.

Scientists ultimately determined that a large outburst from 1ES 1927+654 disrupted the corona enough for it to disappear entirely. Interestingly, the X-ray corona had fully returned by 2021, and 1ES 1927+654 appeared to return to its pre-2018 normal state.

XMM-Newton continued to observe 1ES 1927+654, and in July 2022, scientists noted that its X-ray output was again changing. Specifically, 1ES 1927+654’s X-ray output varied approximately 10% on timescales between 400 and 1,000 seconds, a type of variability called quasi-periodic oscillations (QPO).

“This was our first indication that something strange was going on,” says lead author Megan Masterson of the Massachusetts Institute of Technology.

Scientists believe the oscillations result from a massive object, likely a star, stuck within 1ES 1927+654’s accretion disk. As the star orbits the black hole, its orbital speed increases as it draws nearer to the black hole, increasing the oscillations produced.

Current calculations predict that such an object would likely be a 0.1 solar-mass white dwarf — the leftover core of a low-mass star that has died and burped away its outer layers. Further analysis of XMM-Newton’s observations showed how quickly the white dwarf was orbiting, completing one orbit, or approximately 100 million km, of 1ES 1927+654 every 18 minutes.

XMM-Newton continued to observe 1ES 1927+654 and its white dwarf companion for two years and noted that the oscillations from the black hole were increasing in strength and frequency. Masteron et al. assumed that the orbital energy produced by the white dwarf was being emitted as gravitational waves. The production of gravitational waves coupled with the white dwarf’s velocity around 1ES 1927+654 allowed the team to calculate when the star would cross the event horizon and the oscillations would stop. The team’s results showed that the oscillations should stop in early January 2024.

But, strangely, two months later, in March 2024,  XMM-Newton observations showed that the oscillations had returned. Shockingly, the white dwarf was traveling at half the speed of light and completing an orbit of 1ES 1927+654 every seven minutes. Whatever was inside of 1ES 1927+654 — white dwarf or not — seemingly refused to cross the event horizon and enter the black hole.

Masteron et al. then considered other potential sources for the oscillations. The team remembered the disappearance of 1ES 1927+654’s X-ray corona in 2018 and began investigating whether the corona could be oscillating as a whole. However, with no theory or observed examples of such a phenomenon, the team returned to their original hypothesis and soon found ways to modify it.

Astronomers have discovered binary white dwarf systems throughout the universe, in which two white dwarfs spiral toward each other. Rather than remaining intact as they get closer, the two stars will begin to pull matter from off of each other, which, in turn, can slow down the two objects’  approach. Masteron et al. believe that a similar process may be happening at 1ES 1927+654, wherein the black hole is slowly pulling matter away from the white dwarf.

XMM-Newton cannot make such an observation to confirm whether this new hypothesis is true. However, ESA’s upcoming Laser Interferometer Space Antenna (LISA) mission, currently scheduled to launch in the 2030s, will be able to detect gravitational waves emitted in the exact frequency range that 1ES 1927+654 emits them.

“We predict that if there is a white dwarf in orbit around this supermassive black hole, LISA should see it,” says Masteron.

Infographic showing ESA’s upcoming LISA mission. (Credit: ESA/ATG Medialab)

“This is another great example of XMM-Newton’s unique abilities. It was critical to 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 collecting area across the entire X-ray band, and timing resolution,” XMM-Newton project scientist Norbert Schartel explains.

Masteron et al.’s results are set to be published in the journal Nature in February.

(Lead image: Artist’s depiction of 1ES 1927+654 and its white dwarf companion. Credit: NASA/Sonoma State University/Aurore Simonnet)

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