In the realm of astrophysics, black holes have long captured the fascination of astronomers, researchers, and the public alike. Their enigmatic nature, incredible gravitational pull, and the mysteries surrounding their behavior have led to groundbreaking discoveries and ongoing investigations. One supermassive black hole, 1ES 1927+654, has been at the center of significant attention in recent years, as astronomers have observed a series of unexpected and unprecedented behaviors. These developments are pushing the boundaries of our understanding of the most extreme objects in the universe. The black hole, located approximately 100 million light-years away in a distant galaxy, has exhibited phenomena that continue to astound researchers.
This supermassive black hole, approximately a million times as massive as our Sun, became the subject of intense scrutiny in 2018 when astronomers from MIT and other institutions observed a peculiar event. The black hole’s corona—a cloud of hot, rapidly rotating plasma—disappeared without warning. The corona eventually reassembled months later, but this brief yet dramatic shut-off was a first in black hole astronomy. This mysterious disappearance immediately raised a multitude of questions about the dynamics of black holes and the behavior of plasma near their event horizons, the boundary beyond which nothing, not even light, can escape.
In the years following this startling event, the same team of astronomers continued to monitor 1ES 1927+654. What they found next was even more astonishing. The black hole began emitting flashes of X-rays at an increasingly rapid rate. Over the course of two years, the frequency of these flashes—known as millihertz oscillations—intensified. The time between each burst of X-rays shrank dramatically, from every 18 minutes to every seven minutes. This drastic increase in oscillation frequency was unlike anything astronomers had ever seen before from a black hole.
The increase in the frequency of these X-ray emissions was not merely a curious phenomenon; it presented a new puzzle for the researchers to solve. After ruling out several possible explanations, the scientists hypothesized that the source of the flashing could be a spinning white dwarf—a compact, dead star that has collapsed into a dense core. This white dwarf, they believe, could be orbiting the supermassive black hole and getting dangerously close to the event horizon, the point beyond which it would be irreversibly pulled in. If this hypothesis is correct, the white dwarf would be performing a precarious balancing act, skirting the very edge of the black hole without crossing the point of no return.
Megan Masterson, a graduate student in physics at MIT and one of the lead authors of the study, explained the significance of this scenario. “This would be the closest thing that we know of around any black hole,” she said. “This tells us that objects like white dwarfs may be able to live very close to an event horizon for a relatively extended period of time.” The researchers’ findings were presented at the 245th meeting of the American Astronomical Society, and the results are being published in a paper in Nature as well as on the arXiv preprint server.
The discovery is important not only for its groundbreaking nature but also because it offers a glimpse into the possibility of detecting gravitational waves emitted by the black hole-white dwarf system. These waves, which are ripples in spacetime caused by the acceleration of massive objects, could be detectable by future observatories, including NASA’s Laser Interferometer Space Antenna (LISA), which is slated for launch in the 2030s. Gravitational wave detectors like LISA are specifically designed to detect oscillations occurring on the scale of minutes, which makes 1ES 1927+654’s X-ray oscillations a perfect candidate for study by these next-generation instruments. As Erin Kara, a co-author of the study and associate professor of physics at MIT, pointed out, this system lies within the “sweet spot” for detection by LISA.
The phenomenon that began in 2018 with the black hole’s corona disappearance and continued with the dramatic increase in X-ray flashes has led to renewed interest in 1ES 1927+654. Kara and Masterson were part of the team that observed the first event and felt compelled to continue studying the black hole even as it seemed to be in a relatively stable phase. After the initial coronal shut-off, the black hole’s corona slowly rebuilt itself, and for some time, the object was the brightest X-ray emitter in the sky. Although it showed little variation in behavior for a couple of years, it was still a fascinating object to monitor, and the team was not prepared for the unexpected development that came in 2022.
In that year, the team analyzed new data from the European Space Agency’s XMM-Newton, an advanced space-based observatory that measures X-ray emissions from cosmic sources. They were astounded to find that the black hole’s X-rays appeared to pulse with increasing frequency, a phenomenon known as quasi-periodic oscillations (QPOs). These oscillations are periodic fluctuations in the intensity of X-ray emissions, and they had only been observed in a handful of supermassive black holes prior to this. What made 1ES 1927+654’s oscillations so unique was their rapid acceleration. Over the span of two years, the oscillation frequency sped up from 18-minute intervals to just seven minutes.
Masterson explained the significance of these observations: “We’ve never seen this dramatic variability in the rate at which it’s flashing. This looked absolutely nothing like a normal black hole.” The rapid increase in frequency suggested that the source of the X-ray emissions was situated incredibly close to the black hole, within a few million miles of the event horizon. At such close proximity, the environment is extraordinarily high-energy, where hot plasma moves at relativistic speeds, generating intense X-rays.
The X-ray emissions from 1ES 1927+654 also prompted the team to consider the possibility that the source was not the black hole’s corona but rather an object orbiting very close to the event horizon. The innermost regions of a black hole’s accretion disk, where matter spirals in toward the black hole, are known to generate X-rays due to the extreme conditions. However, X-rays are less likely to be emitted from regions farther away, where the gas moves more slowly, emitting primarily optical and ultraviolet light.
Masterson and Kara turned to models of various astrophysical phenomena to explain the X-ray variability. One theory considered oscillations in the black hole’s corona, where the shrinking of the corona could lead to faster oscillations. However, the team favored another explanation: the presence of a white dwarf.
White dwarfs are incredibly dense objects, the remnants of stars that have exhausted their nuclear fuel. The research team’s modeling suggested that the white dwarf could be about one-tenth the mass of our Sun, in contrast to the supermassive black hole, which is approximately a million times the mass of the Sun. The proximity of the white dwarf to the black hole would cause it to move in a rapidly increasing orbit, which could explain the accelerating X-ray flashes. The closer the white dwarf gets to the event horizon, the faster it would need to orbit to maintain its position, which would result in an increase in the frequency of oscillations.
At this distance, gravitational waves would also be produced, potentially detectable by LISA in the future. The white dwarf is so close to the black hole’s event horizon that it is effectively at the edge of no return, but the researchers believe that the star will not actually fall into the black hole. As the white dwarf approaches the event horizon, it is losing mass through a process called tidal stripping, where the black hole pulls away part of the star’s outer layers. This process could give the white dwarf just enough momentum to resist being consumed by the black hole. Because white dwarfs are compact and resilient, they may be able to survive at such extreme proximity without being completely destroyed.
Kara emphasized the remarkable nature of the discovery, saying, “Because white dwarfs are small and compact, they’re very difficult to shred apart, so they can be very close to a black hole.” The white dwarf in question may be at the brink of crossing the event horizon but could potentially resist falling in, offering a rare opportunity to observe an object in such close proximity to a black hole without it being destroyed.
The team plans to continue monitoring 1ES 1927+654 with existing telescopes and future instruments like LISA, as the black hole’s behavior is expected to provide new insights into the physics of black holes, gravitational waves, and the interactions between compact objects and supermassive black holes. As Masterson concluded, “The one thing I’ve learned with this source is to never stop looking at it because it will probably teach us something new. The next step is just to keep our eyes open.” The mysterious behavior of 1ES 1927+654 is far from over, and its study promises to be a key milestone in the quest to understand the most extreme environments in the universe.
Reference: Megan Masterson et al, Millihertz Oscillations Near the Innermost Orbit of a Supermassive Black Hole, arXiv (2025). DOI: 10.48550/arxiv.2501.01581