Mysterious x-ray flashes are accelerating in a black hole

Astronomers have been captivated by the unusual activities of the supermassive black hole 1ES 1927+654. Located about 100 million light-years away, this particular black hole has a mass that is equivalent to roughly one million Suns. 

Initially, in 2018, researchers witnessed a striking event when the black hole‘s corona – a cloud of whirling, white-hot plasma – vanished unexpectedly, only to reassemble months later.

This dramatic “shut-off” was a first in the study of black holes, setting the stage for further intriguing events.

X-ray flashes from a black hole

More recently, the same black hole has displayed another baffling phenomenon. Over a span of two years, astronomers detected X-ray flashes from 1ES 1927+654 that increased in frequency from one flash every 18 minutes to one every seven minutes. This dramatic speed-up in X-ray oscillations is unprecedented for black holes.

The research team explored various explanations for these flashes and leaned toward a scenario involving a white dwarf. 

They propose that a spinning, compact core of a dead star – a white dwarf – is orbiting dangerously close to the black hole’s event horizon. 

The white dwarf hypothesis

This white dwarf appears to be approaching the boundary where nothing can escape the black hole’s gravitational pull while remarkably avoiding being consumed.

“This would be the closest thing that we know of around any black hole,” said lead author Megan Masterson, a graduate student in physics at MIT. “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 scientists suggest that the white dwarf, estimated to be about one-tenth the mass of the Sun, is engaged in a precarious orbit around 1ES 1927+654. Its intense proximity is inferred from the observed X-ray variability, which points to processes occurring near the black hole’s edge.

Flashes from a death-defying star

As the white dwarf orbits, it would emit gravitational waves, drawing it ever closer to the black hole. This tightening orbit would naturally lead to an increase in the frequency of X-ray flashes.

Remarkably, the white dwarf may not actually fall into the black hole despite its close approach. 

The researchers theorize that as the white dwarf sheds part of its outer layers, this process provides a counteracting force that prevents the star from crossing the point of no return.

Gravitational waves and future observations

If the white dwarf scenario is correct, the system is expected to emit gravitational waves at frequencies detectable by instruments like NASA’s upcoming Laser Interferometer Space Antenna (LISA). 

“These new detectors are designed to detect oscillations on the scale of minutes, so this black hole system is in that sweet spot,” said Erin Kara, an associate professor of physics at MIT and co-author of the study.

The detection of such gravitational waves would offer a new way to study the extreme physics near black holes and confirm the presence of the white dwarf.

Dramatic variability in the black hole flashes

Researchers have never before observed such dramatic variability in the X-ray emissions of a supermassive black hole. 

“We’ve never seen this dramatic variability in the rate at which it’s flashing,” Masterson said.

The frequent and escalating X-ray pulses suggest that the phenomenon is occurring very close to the black hole, where high-energy processes dominate and where objects must move at incredible speeds due to strong gravitational forces.

The team examined data from the European Space Agency’s XMM-Newton observatory, which specializes in capturing X-ray emissions.

The discovery of quasi-periodic oscillations in the X-ray light from this black hole indicates that the source of these emissions is likely in the innermost regions near the event horizon.

Extreme environments near supermassive black holes

While one possible explanation for the X-ray patterns was oscillations in the black hole’s corona, the team currently favors the white dwarf hypothesis due to a better grasp of the underlying physics. 

The researchers plan to continue monitoring 1ES 1927+654 with current telescopes and prepare for future observations using LISA.

Masterson emphasized the need for continuous observation. “The one thing I’ve learned with this source is to never stop looking at it because it will probably teach us something new.” 

As gravitational wave detectors are built, they may capture signals from this extraordinary system, providing deeper insights into the extreme environments near black holes.

Understanding the physics of black holes

The behavior of 1ES 1927+654 offers a rare opportunity to study the dynamics of objects in extreme gravitational fields.

If a white dwarf is indeed orbiting close to the event horizon, this system will serve as a unique laboratory for understanding how matter behaves under such intense conditions. 

The increasing frequency of X-ray flashes and the potential detection of gravitational waves could revolutionize our understanding of black hole physics and the life cycles of compact objects in their vicinity.

A preprint of the article – which has been accepted by the journal Nature – has been published in the arXiv database.

Image Credit: Aurore Simonnet/Sonoma State University

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