One supermassive black hole has been keeping astronomers in their sights for the past few years. First, there was an unexpected disappearance, and now there is a dangerous spinning act. The black hole in question is 1ES 1927+654, which is as massive as a million suns and is located in a galaxy 100 million light-years away. In 2018, astronomers at the Massachusetts Institute of Technology and elsewhere noticed that the black hole’s corona—a cloud of white-hot plasma—suddenly disappeared, then reassembled a few months later. The brief but dramatic outage was a first in black hole astronomy.
Now, a team from the Massachusetts Institute of Technology has discovered the same black hole, exhibiting unprecedented behavior. Astronomers have recorded bursts of X-ray radiation coming from the black hole, steadily increasing. Over the course of two years, the frequency of the millihertz bursts increased from every 18 minutes to every seven minutes. Such a dramatic acceleration of X-ray radiation has never been observed from a black hole.
Researchers have explored several scenarios that could explain the flares. They believe the most likely culprit is a rotating white dwarf — the extremely compact core of a dead star that orbits a black hole and is precariously approaching its event horizon, the boundary beyond which nothing can escape the black hole's gravitational pull. If so, the white dwarf must be performing an impressive balancing act, as it could have walked right up to the edge of the black hole without actually falling in.
“This would be the closest we know to any black hole,” says Megan Masterson, a graduate student in physics at MIT who co-led the discovery. “It tells us that objects like white dwarfs can live very close to the event horizon for relatively long periods.”
The researchers presented their findings at the 245th meeting of the American Astronomical Society in National Harbor, Maryland, and will publish the results in a paper in Nature The findings are also published on the preprint server. arXivIf a white dwarf is at the heart of the mysterious black hole flare, it is also emitting gravitational waves in a range that could be detected by next-generation observatories such as NASA's Laser Interferometer Gravitational-Spatial Array (LISA).
“These new detectors are designed to detect minute-scale fluctuations, so this black hole system is in this prime spot,” says co-author Erin Kara, an associate professor in the Department of Physics at MIT. Other co-authors on the study include MIT Kavli Fellows Christos Panagiotu, Johin Chakraborty, Kevin Burge, Riccardo Arcodia, Ronald Remillard and Jingyi Wang, as well as collaborators from many other institutions.
Nothing normal
Kara and Masterson were part of a team that observed 1ES 1927+654 in 2018, as the black hole's corona dimmed and then slowly recovered over time. For a time, the newly transformed corona — a cloud of high-energy plasma and X-rays — was the brightest X-ray object in the sky.
"He was still extremely bright, even though he hadn't done anything new for a few years and was kind of bubbling over. But we felt like we had to keep following him because he was so beautiful," says Kara. "Then we noticed something we'd never really seen before."
In 2022, the team analyzed observations of the black hole made by the European Space Agency's XMM-Newton, a space observatory that detects and measures X-ray emission from black holes, neutron stars, galaxy clusters, and other extreme cosmic sources. They noticed that the X-ray emission from the black hole was pulsating with increasing frequency.
Such "quasi-periodic oscillations" have only been observed in a few other supermassive black holes, where X-ray flares appear at regular intervals. In the case of 1ES 1927+654, the flickering seemed to increase steadily, from every 18 minutes to every seven minutes over two years.
"We've never seen such dramatic variability in the rate of blinking," Masterson says. "It didn't look like a normal black hole at all."
The fact that the flare was detected in the X-ray range indicates a high probability that the source is somewhere very close to the black hole. The inner regions of a black hole are extremely high-energy environments where X-rays are produced by fast, hot plasma.
X-rays are less often seen at greater distances, where the gas can rotate more slowly in the accretion disk. The cooler disk environment may emit optical and ultraviolet light, but rarely emits X-rays.
“Seeing something in X-rays means you’re pretty close to the black hole,” says Kara. “When you see variability on a time scale of minutes, that’s close to the event horizon, and the first thing you think about is circular motion and whether something could be orbiting the black hole.”