Black Hole Sprouts Jets in Unprecedented Event – Sky & Telescope

Seven years ago by our clocks, the black hole at the center of a galaxy some 200 million light-years away ate a quick meal. That act kicked off a series of events that may have changed its appearance for millennia to come, a team of astronomers reported at the 245th meeting of the American Astronomical Society in Washington, D.C.

Over the course of a few months, the galaxy 1ES 1927+654 suddenly became 100 times brighter at visible wavelengths than it was before, catching the attention of an automated search for supernovae. But this was no exploding star. The light was coming from material whirling around and into the galaxy’s supermassive black hole, which weighs in with the equivalent of 20 million Suns.

Astronomers at the time suspected that, to produce the burst of light, the black hole must have suddenly gulped down a glut of gas — perhaps siphoned off from a star torn apart in the black hole’s extreme gravitational field. Such events are rare, especially around supermassive black holes, and multiple teams followed up, observing the galaxy using X-ray space telescopes and ground-based radio telescopes.

X-ray Restart

The next surprise came in late 2018, with a sudden, extreme drop in X-rays. “What was producing the majority of the X-ray emission, in particular the high-energy X-rays, was destroyed,” Megan Masterson (MIT) said during the AAS press conference. These high-energy X-rays come from the corona, an X-ray-emitting region associated with feeding black holes. It took months for the X-ray emission to climb back to baseline. “We have basically watched the corona reform in real time,” Masterson added.

But by the time the X-ray emission returned, its nature had changed. Whereas before the emission had twinkled in a random way, now the brightness varied periodically. In 2022 that period was roughly 18 minutes, and by 2024 it had dropped to 7 minutes.

One explanation is that the signal comes from something that’s still orbiting the black hole, and that the periodic variation corresponds to the time the thing takes to complete a circuit. If so, then the source of the signal is circling the black hole at the distance Mercury orbits the Sun — equivalent to twice the radius of the black hole’s event horizon, the point of no return. (To be clear, that’s really, really close.)

The team speculates that the orbiting thing could be a white dwarf, the leftover core of a Sun-like star. The dwarf would be compact enough to survive such a close encounter with the black hole without being instantly shredded, although it’s likely losing some gas to the leviathan’s gravity in a bid to slow its fall into the maw.

It’s unclear, though, how the putative presence of a white dwarf could be related to the original outburst — in fact, it may well not be. The white dwarf might have been in orbit around this black hole all along, and an unconnected outburst just happened to go off at the right time for us to see the white dwarf’s quickly decaying orbit.

What excites Masterson and her colleagues about the white dwarf possibility is that the Laser Interferometer Space Antenna (LISA) mission, due to launch in the mid-2030s, could actually detect gravitational waves from this system and confirm the scenario — if the white dwarf lasts long enough.

Birth of a Radio-emitting Jet

The years-old outburst had larger effects as well: In early 2023, about 200 days after the X-rays started to recover, radio emission from near the black hole suddenly ratcheted up from nothing. “We’ve never had this happen,” Eileen Meyer (University of Maryland, Baltimore County) said during the press conference. “We’ve never been looking at a black hole and watched it go from being radio quiet to suddenly very radio loud.”

That radio emission comes from a pair of jets of hot gas, which are shooting away from the black hole at a third the speed of light. “Initially, there’s nothing, and then there’s this emergence of these blobs, which is the hot gas being emitted on either side of the black hole,” Meyer explained.

Because the black hole probably quaffed a single meal rather an ongoing feast, these jets are still small and likely short-lived, akin to other stubby jet systems called compact symmetric objects, Meyer’s team speculates in a study published in the January 20th Astrophysical Journal Letters. If that’s so, then the jets might last at most 1,000 years. If the material keeps flying out at its current speed, the jets will only extend some 300 light-years before they turn off.

The jets’ presence offers another possibility for the X-ray signal: The base of the jet itself could be what’s oscillating, eliminating the need for a white dwarf companion. That explanation is tempting, if only to understand all the observed phenomena in one fell swoop. The black hole might have eaten a star, an act that disrupted X-ray emissions, and then burped out some plasma jets that produced both oscillating X-rays and radio waves. Another point in favor of this scenario is that the overall X-ray brightness appears to be changing as the period of brightness variations falls.

The sticking point is that there’s no obvious way to explain why the base of a jet should oscillate so quickly — this is part of why the team prefers the white dwarf option. The white dwarf is also a testable hypothesis with the launch of LISA, whereas the oscillating jet scenario has no ready test.

“The fun thing about is we really don’t know what it’s going to do,” Meyer says. “It may continue to be exactly this bright for the rest of my life, and it may turn off tomorrow. Certainly we’ll learn something when that happens.”