In 2018, a distant black hole threw a fit. The 1.4-million-solar-mass black hole at the center of the galaxy 1ES 1927+654 some 270 million light-years away suddenly began spewing radiation, flaring in optical, ultraviolet, and X-ray light.
Then, astronomers watched as the so-called X-ray corona of high-energy particles close in to the black hole vanished — only to slowly reform over time. The most likely cause of this strange behavior was a wayward star wandering too close, only to be torn apart and consumed by the black hole in what astronomers call a tidal disruption event, or TDE.
Since then, researchers have kept a close eye on this target. And their vigilance has paid off in two big ways, astronomers announced yesterday at the 245th meeting of the American Astronomical Society in National Harbor, Maryland.
A jet is born
For a while after the TDE, 1ES 1927+654 was quiet, particularly at radio wavelengths. But then, in 2023, researchers saw its X-ray output begin increasing. This prompted University of Maryland Baltimore County astronomer Eileen Meyer and her team to check in on the galaxy’s radio emission. They discovered that the black hole was flaring at that wavelength, too. At its peak, she said in her presentation, the galaxy’s radio emission spiked to more than 60 times its previous level and remained high.
“Normally, this level of radio emission would tell us that there are jets,” she said — a statement corroborated by a subsequent year of high-resolution radio images centered on the supermassive black hole and showing details smaller than a light-year. They show two blobs of hot gas moving away from the supermassive black hole at some 33 percent the speed of light.
“This is unprecedented. We’ve never had this happen. We’ve never been looking at a black hole and watched it go from being radio quiet to suddenly very radio loud,” Meyer said. Essentially, her team witnessed the birth of jets from a supermassive black hole, in real time.
Meyer is lead author on a paper outlining the discovery, published Jan. 13 in The Astrophysical Journal Letters.
Quick development
Just 10 percent of supermassive black holes are shooting out radio jets, typically spanning thousands of light-years or more and reaching out into intergalactic space. But while we have watched such jets evolve over time, we’ve never seen one actively form from nothing.
Further, astronomers have long thought that forming jets takes a long time: “We expect that the timescales for these things to turn on and turn off are long — and certainly longer than, say, a human lifetime,” Meyer said. “Or at least, that was kind of our naive expectation earlier on.”
The team thinks in this case, the jets likely formed due to to the TDE observed in 2018. And perhaps this signals that 1ES 1927+654 is a type of “scaled-down” situation classified as a compact symmetric object, which turns on for thousands of years rather than millions, and is triggered not by a huge, ongoing influx of material, but by a single TDE.
Following X-ray flashes
Forming jets aren’t the only reason 1ES 1927+654 is now in the spotlight. Following Meyer’s talk was Megan Masterson, a graduate student at MIT and lead author of a paper now accepted for publication in Nature and available on the arXiv preprint server.
Masterson focused on the X-rays seen from 1ES 1927+654, which at first exhibited the expected random variations typically seen from feeding black holes following the 2018 event. But beginning in 2022, the X-ray emission changed, displaying periodic oscillations as the X-rays brightened and faded with a regular period of about 10 minutes. Such short-timescale oscillations, she said, are extremely hard to detect, and 1ES 1927+654 is one of only a handful of supermassive black holes in which they have been seen.
Then, something changed. The period of the X-rays began shrinking dramatically: In 2022, the period was 18 minutes. In 2024, it was seven minutes, and seems to have stabilized there.
Two possibilities
What could create regular X-ray “flickers” so close to a supermassive black hole? And why is the period shrinking? There are two possibilities, Masterson said. One is that the flickers are related to the newly formed jet found by Meyer’s team. If the X-rays are coming from the base of the jet, oscillations in the jet itself or changes in the physical size of the base of the jet could account for the changing period.
But there’s an even more intriguing possibility. “Orbits naturally give you this nice periodic behavior. And … there’s a really easy way to get your period to change when you’re in orbit, and that’s to change the actual distance of that orbit,” she said.
So, some object may be orbiting ever-closer to the supermassive black hole — a phenomenon that has been seen before, she said, though other such orbiters have periods of hours or days, not minutes. In the case of 1ES 1927+654, a seven-minute period translates to an orbit just a few million miles from the event horizon, the point of no return, “on the edge of that accretion disk,” around the black hole, Masterson said.
The orbiter “cannot be a [smaller] black hole,” because a stellar-mass black hole orbiting so close would quickly lose energy by emitting gravitational waves and plunge into the bigger black hole. It wouldn’t keep orbiting for years. “Instead … we need some additional source of energy, in order to be able to keep this object outside the black hole’s event horizon,” she said.
That’s why her team thinks the orbiter is a 0.1-solar-mass white dwarf — the superheated, compact remnant of a Sun-like star. A white dwarf would also lose angular momentum through gravitational waves, but it also has an additional source of energy. By slowly losing its outermost material to the supermassive black hole in a process called mass transfer, a white dwarf could gain energy and angular momentum, keeping it in orbit rather than spiraling in.
Why a white dwarf specifically, and not just any star? “White dwarfs are small and compact, they’re very difficult to shred apart, so they can be very close to a black hole,” explained MIT astronomer Erin Kara, a co-author on the paper, in a press release.
Waiting to hear
For now, it’s not clear which scenario is correct. But when ESA’s Laser Interferometer Space Antenna (LISA) launches in the 2030s, it will the sensitivity to detect gravitational waves with frequencies in the range of what astronomers expect to be coming from 1ES 1927+654, if there is an orbiting white dwarf. And if LISA doesn’t “hear” anything, then the more likely cause of the flickering X-rays is the jet, which wouldn’t produce gravitational waves.
For now, astronomers will continue to closely watch 1ES 1927+654. “The one thing I’ve learned with this source is to never stop looking at it because it will probably teach us something new,” Masterson said.
