Astrophysics

How Black Holes Shoot Out Lethal Plasma Jets Is Becoming Clearer

Researchers are trying to understand the extreme physics of black holes, and are zeroing in on how they project bright columns of hot gas and radiation.

This artist's concept shows a black hole with an accretion disk — a flat structure of material orbiting the black hole — and a jet of hot gas, called plasma. | NASA/JPL-Caltech
This artist's concept shows a black hole with an accretion disk — a flat structure of material orbiting the black hole — and a jet of hot gas, called plasma. | NASA/JPL-Caltech

Did you know that black holes will occasionally shoot spectacular dual jets of hot gas out into intergalactic space? It's one of the ways we know that black holes really do exist. But because astronomers can’t see the inner workings of black holes, they haven’t yet figured out how these plasma jets form.

Observations from several observatories of a black hole jet event in 2015 are providing new clues about where the jets form and how they get energized enough to radiate in multiple wavelengths as two bright columns shooting along the black hole's axis of rotation. Using a new quick and sharp-eyed camera called ULTRACAM on the William Herschel Observatory in La Palma, Spain, and the NuSTAR X-ray space telescope, astronomers have been able to measure the distance that particles in jets travel before they “turn on” and become bright sources of light.

“Multiple telescopes working in tandem were crucial for this work,” astronomer Poshak Gandhi from the University of Southampton in the UK told Seeker in an email. He’s the lead author of a new study published this week in the journal Nature Astronomy. “We needed X-ray 'eyes', optical 'eyes' as well as radio eyes to isolate and unambiguously associate the fast flaring with the base of the jet.”

These jets have been fancifully compared to rays blasting from the Death Star in the Star Wars films. They produce high amounts of radiation, especially high-energy X-rays and gamma-rays, which can be lethal for anything in the path of the beams.

Gandhi explained that the current theory of how the jets form is that infalling material made of hot gas plasma swirls around the black hole in what’s called an accretion disk, very close to the black hole.

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“This plasma is hot with characteristic temperatures of hundreds of millions of degrees and more; hot enough to emit X-rays that we saw from NuSTAR,” he said. “Most of this material falls into the black hole. But some of it can instead be funneled outwards by strong magnetic fields. This is the place where the jet forms, probably very close to the black hole. How exactly this material is flung out and accelerated in the jet is what we are ultimately trying to probe.”

The astronomers observed a bright jet event in a binary system called V404 Cygni. This system consists of a star slightly smaller than the sun that orbits a black hole 10 times its mass in only 6.5 days. The close orbit and strong gravity of the black hole produce tidal forces that pull a stream of gas from the star.

The gas travels to the accretion disk, heating up to millions of degrees. At some point, between the heat and churning magnetic fields, the swirling particles in the accretion disk are flung out at close to the speed of light, creating the jets.

This system has been known to flare before, but it has been quiet since 1989. In June of 2015  V404  Cygni glowed with one of the brightest outbursts of light from a black hole ever seen, and it immediately caught the attention of several telescopes.

Using telescopes on Earth and in space observing at exactly the same time, astronomers were able to discern a tiny 0.1-second delay between X-ray flares emitted from near the black hole, where the jet forms, and the appearance of visible light flashes.

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The X-ray emission, representing the accretion disc “feeding” the jet at its base, was captured from Earth orbit by the NuSTAR telescope, while the moment the jet became visible as  optical  light was caught by the ULTRACAM high-speed camera.

“Our data on V404 Cygni tell us that the jet does not necessarily start to glow strongly immediately,” Gandhi explained. “Instead, it emits optical light at a distance of about 0.1 light-seconds, which is probably a bit under 30,000 km (19,000 miles) depending upon the exact speed of the plasma.”
Why does it glow in this staggered fashion?

“Plasma acceleration is likely to be haphazard and non-uniform,” Gandhi said. “Some chunks of plasma will be accelerated more than  others,  and will crash into each other in a turbulent manner. This is somewhat like the turbulence between different layers of air in the atmosphere that cause a bumpy plane ride; just far more extreme. This turbulence is enough that plasma is 'shocked' and begins to glow, radiating  away  this energy as optical light.”

While this event provides new insights into the jets, Gandhi said they don’t have all the answers yet to explain what happens in the split second in the “acceleration zone” where the X-rays turn on in optical light.

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“The new observations will help to refine the theories that can answer this,” he said. “But all this remains to be tested in detail. The essential point is that we now have a quantitative measurement of the location in the jet where this occurs, hence a measurement of the size of the jet over which the plasma has been strongly accelerated. This information can be fed into various models to refine our understanding of jet physics.”

Making these types of coordinated observations is not easy. The X-ray telescopes in space and optical telescopes on the ground have to look at the X-ray binaries at exactly the same time during outbursts for scientists to calculate the tiny delay between the telescopes' detections.

“It's been very difficult historically to coordinate such observations from telescopes working simultaneously around the globe and in space,” Gandhi explained. “This is a complex logistical challenge, requiring advance preparation and operational flexibility. Moreover, the technology to observe the fast optical flashes is still pretty new.”

For the jet event in 2015, the coordination between NuSTAR and ULTRACAM was only possible for about an hour. But the team said that was enough time to calculate the groundbreaking results about the acceleration zone.

“The event that we saw in 2015 was an outstandingly bright ‘outburst’ of radiation from this black hole system V404 Cygni,” Gandhi said, “which prompted astronomers around the world to work together, resulting in the discovery that we made. It was hard work by a big team of people.”

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