Astronomers are grappling with a complex cosmic mystery lurking at the dark heart of a distant galaxy some 270 million light-years from Earth. And its resolution could revolutionize our understanding of how black holes feast on matter throughout the universe.
Known as 1ES 1927+654 and located in the constellation Draco, this far-off island of stars harbors at its core a supermassive black hole weighing more than a million suns—which, surprisingly, isn’t very remarkable. Most large galaxies, including our own, host such hefty monstrosities at their center. But this black hole has proved extraordinarily strange: the object shocked observers with an abrupt outburst of radiation so intense that it apparently obliterated the black hole’s corona, an enveloping cloud of whirling, billion-degree plasma, for three months in 2018. The outburst, it was thought, could’ve come from a tidal disruption event, which occurs when an unlucky star is torn apart and devoured after drifting too close to a black hole. Many research groups began closely monitoring the system, watching across the next few years as the corona reassembled itself and quiescent conditions returned, until the black hole unleashed more surprises—dramatically flaring in radio waves and flickering with rapid pulses of x-rays.
Such a dizzying assortment of dynamic activity is unprecedented around a supermassive black hole and can’t be readily explained by any typical tidal disruption event. Eileen Meyer, an astronomer at the University of Maryland, Baltimore County, who led an international team in the investigation of the system’s radio emissions with the use of multiple telescopes on the ground and in space, recalls her initial impression of 1ES 1927+654 as that of a very “boring, faint radio blob.” But as she and her colleagues saw more and more strange activity unfold, she realized “this [black hole] was weird, very weird.” In particular, her team’s observations revealed that shortly after its radio wave flare-up, the black hole had belched out a pair of giant, oppositely directed plasma jets traveling at one third the speed of light. This was the first time the creation of such jets had been witnessed in real time, and it was a clear indicator of extreme activity closer in to the black hole. Meyer presented her team’s findings last week at the 245th meeting of the American Astronomical Society in National Harbor, Md., and was lead author on an accompanying paper published January 13 in the Astrophysical Journal Letters.
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If a run-of-the-mill act of stellar destruction wasn’t behind the black hole’s bizarre behavior, what was? A crucial clue could lie in the timing of the black hole’s flickering x-ray pulses, which were unveiled via work led by Megan Masterson, a doctoral candidate at the Massachusetts Institute of Technology. Using data from the European Space Agency’s XMM-Newton x-ray space telescope, Masterson and her colleagues found a telltale pattern in the pulses: an oscillation in their flickering that became faster and faster over two years. “The period of this oscillation changed dramatically from 2022 to 2024,” Masterson says. “We started out in 2022 at an 18-minute period, and by 2024, we were at a seven-minute period—so the period has basically been cut in half. That has never been observed around a supermassive black hole before.” A paper reporting the results, co-authored by Masterson, Meyer and others, was posted to the preprint server arXiv.org in January and has been accepted for publication in the February 13 edition of Nature.
The most obvious explanation for these x-ray oscillations, the researchers say, is that they’re a clear but indirect signal of a substantial something orbiting very close to the black hole. It’s so close, in fact, that it must be plowing through the black hole’s accretion disk—a maelstrom of infalling matter made incandescent from frictional heating as it piles up around the gravitational monster’s maw. If that were the case, the researchers realized, each flickering eruption would correspond to the object completing one orbital cycle, upon which it would swoop through and agitate the accretion disk to kick out a burst of x-rays. And the curious quickening of the oscillations appeared to be a sign that this object’s orbit was decaying, bleeding off energy and spiraling ever closer and faster toward a point of no return—the black hole’s event horizon—via the emission of ripples in spacetime called gravitational waves.
For Masterson, the next step was simple: “I calculated how long it’s going to take that body to inspiral and be eaten,” she says. The math told Masterson that the hypothetical object’s final plunge would occur in January 2024. Then, at last, the mysterious x-ray oscillations would stop.
But they didn’t. XMM-Newton observations of 1ES 1927+654 from March 2024 clearly showed that the oscillations were still going strong; if caused by an orbiting object, their roughly seven-minute period meant the black hole’s companion was within a few million miles of the event horizon and moving at half the speed of light. No object has ever been observed so close to a black hole; why wasn’t this one falling in? Gravity should have ensured its doom—unless something other than gravity was at play here, Masterson remembers thinking. And she found one promising candidate in another unexpected area: the physics of white dwarfs, which are compact stellar corpses left behind by dying sunlike stars.
If the putative object were a smaller black hole, it would’ve plunged headlong through the accretion disk to merge with its supermassive mate—and if it were a normal star, it should’ve been shredded upon its close approach to produce a typical tidal disruption event. But, Masterson and her team realized, if it were a low-mass white dwarf, about the same size as Earth, it could be hardy enough to precariously perch for a time on the edge of destruction. Rather than succumbing to star-shredding tidal forces, such a white dwarf would instead trickle-feed a small fraction of its matter to the black hole. This could offset the orbital energy being lost through gravitational waves, halting or even reversing the inspiral. “Essentially it’s something special that has to do with how a white dwarf responds to losing its mass and how accretion physics play in,” Masterson says.
This makes sense, says Chiara Mingarelli, an astrophysicist at Yale University, who was not involved in either study of 1ES 1927+654. If the hypothetical object orbiting the black hole were a white dwarf, the undead star would be in some sort of tidal limbo, where it would be “starting to get ripped apart a little bit,” she explains, “emitting gravitational waves [while] slowly spiraling into the black hole instead of just being gobbled up whole.”
Nevertheless, this model remains at best an educated guess. Its validation could come relatively soon, however, via a space-based gravitational-wave detector set to launch in the 2030s: the European Space Agency’s Laser Interferometer Space Antenna, or LISA, which should be able to detect the gravitational waves pouring off a white dwarf in a state of quasi-stasis around 1ES 1927+654. And even if LISA finds no such signature, that null result should help solve the mystery of what’s really happening in this enigmatic system—perhaps, for instance, the radio flares, giant jets and x-ray pulses all instead trace back to poorly understood interactions between the black hole and its vanishing and reappearing cloud of coronal plasma.
Either way, “it’s an opportunity for us to study this one source right now, and hopefully LISA will find many, many more [cosmic systems that are similar], and then we can study all of them,” Masterson says.
“I was surprised and delighted that there’s so much left for us to understand about black hole dynamics, especially accretion-disk physics,” Mingarelli says, adding that the potential of LISA to study these environments could unlock many more mysteries about supermassive black holes.
“It’s not just about observing the static universe anymore,” Meyer says. “Now we’re at the point where we realize how much of the universe is very dynamic—we don’t know what’s coming up. There might be something new there that wasn’t there last week.”
