Astronomers have observed unusual behavior in a supermassive black hole in a galaxy around 100 million light-years away: measurements from the European X-ray satellite XMM-Newton show that this black hole has been exhibiting faster oscillations in the X-rays it releases since 2022. The timing of these X-ray oscillations suggests that their source is very close to the black hole’s event horizon. It is possible that behind these oscillations there is a white dwarf orbiting closely around the black hole, as astronomers report. What is unusual, however, is that this remnant of stars is attracted to the black hole far more slowly than expected for an object of its mass.
If a star or a gas cloud gets too close to a supermassive black hole, its fate is sealed: it is torn apart by the enormous tidal forces surrounding the black hole and the heated and accelerated material forms an accretion disk orbiting around the event horizon, from which high-energy UV – and X-rays go out. This so-called “tidal disruption event” usually becomes visible through a radiation burst that can be seen from afar.
First breakout, then oscillations
Astronomers also detected such an event in March 2018 around the supermassive black hole 1ES 1927+654. This black hole, which weighs around a million solar masses, lies at the center of a galaxy around 100 million light-years away. Observations with the European Space Agency’s XMM-Newton X-ray satellite first showed a burst of radiation, then a disappearance of the pre-existing corona of hard X-rays around the object. “The radiation flux of X-rays in the range of 0.3 to 10 kiloelectron volts fell by a factor of 1000 within two months before gradually increasing again,” report Megan Masterson from the Massachusetts Institute of Technology (MIT) and her colleagues. This suggested that the accretion disk and the surrounding X-ray corona had been temporarily decimated during the eruption. It was not until 2021 that the X-ray emissions around the active black hole returned to their previous intensity.
“Nothing new happened for a few years, but we kept an eye on it because the X-ray corona was so beautiful and bright,” reports co-author Erin Kara from MIT. “But then we noticed something that had never been observed before.” In 2022, data from XMM-Newton showed that the X-rays around the black hole 1ES 1927+654 fluctuated at an almost regular rate. These oscillations occurred on average every 18 minutes. “So far, only a handful of accreting supermassive black holes are known to exhibit such quasi-periodic oscillations in the millihertz range,” the astronomers write. What was even more unusual, however, was that the interval between these oscillations decreased to just seven minutes within just two years. “We have never seen such dramatic variability in the rate of bursts of radiation before,” says Masterson. “This is very different from a normal black hole.”
What is the cause of X-ray fluctuations?
The astronomers suspect that these X-ray fluctuations are caused by an object orbiting rapidly around the event horizon of 1ES 1927+654. “When you see a phenomenon like this in the X-ray region, it tells you that there is something very close to the black hole,” explains Kara. In this case, astronomers estimate the distance to be just a few million kilometers from the event horizon. It could be an extremely compact object with the mass of a star that is currently on a spiral death course into the black hole. However, another feature of the oscillations does not fit into the picture: Typically, the oscillations would have to accelerate faster the closer the object gets to the event horizon. But the X-ray oscillations at the black hole 1ES 1927+654 increase more and more slowly over time. According to Masterson and her team, the object must therefore release material without being destroyed. This mass transfer gives it a little “kick” that slows down and perhaps even prevents its fall into a black hole.
Using model simulations, the astronomers identified the most likely candidate for such an object as a white dwarf with around a tenth of the mass of the sun. Such an extremely dense, small stellar remnant would be stable enough to remain intact despite loss of material from its outer shell. “Because these stellar remnants are so small and compact, they are difficult to tear apart, allowing them to get very close to a black hole without being destroyed,” explains Kara. “This could then be the closest object ever detected to a black hole.” Theoretically, it would even be possible for this white dwarf to not only slow its fall into the black hole, but even avoid it entirely. However, it is still unclear whether there really is a white dwarf in orbit around the supermassive black hole 1ES 1927+654 or whether the unusual X-ray oscillations have another, as yet unknown, cause. The astronomers hope that further observations, including with the gravitational wave detector LISA, which will be launched into space in the 2030s, will provide more clarity.
Source: Megan Masterson (Massachusetts Institute of Technology (MIT), Cambridge, USA) et al., Nature, doi: 10.48550/arXiv.2501.01581