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How Extreme Energy Jets Escape a Black Hole

Researchers are using XSEDE supercomputers to better understand the forces at work at the center of the Milky Way galaxy. The work could reveal how instabilities develop in extreme energy releases from black holes.

Some of the most extreme outbursts observed in the universe are the mysterious jets of energy and matter beaming from the center of galaxies at nearly the speed of light. These narrow jets, which typically form in opposing pairs are believed to be associated with supermassive black holes and other exotic objects, though the mechanisms that drive and dissipate them are not well understood.

Now, a small team of researchers has developed theories supported by 3-D simulations to explain what’s at work.

These jets are notoriously hard to explain,” said Alexander “Sasha” Tchekhovskoy, a former NASA Einstein fellow who co-led the new study as a member of the Nuclear Science Division at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), and the Astronomy and Physics departments and Theoretical Astrophysics Center at UC Berkeley. “Why are they so stable in some galaxies and in others they just fall apart?”

As much as half of the jets’ energy can escape in the form of X-rays and stronger forms of radiation. The researchers showed how two different mechanisms – both related to the jets’ interaction with surrounding matter, known as the “ambient medium” – serve to reduce about half of the energy of these powerful jets.

The exciting part of this research is that we are now coming to understand the full range of dissipation mechanisms that are working in the jet,” no matter the size or type of jet, he said.

An animation showing magnetic field instabilities in two jets of radiation and matter beaming from a supermassive black hole (center). The magnetic field (white) is twisted by the black hole’s spin. (Credit: Berkeley Lab, Purdue University)

The study that Tchekhovskoy co-led with Purdue University scientists Rodolfo Barniol Duran and Dimitrios Giannios is published in the Aug. 21 edition of Monthly Notices of the Royal Astronomical Society. The study concludes that the ambient medium itself has a lot to do with how the jets release energy.

We were finally able to simulate jets that start from the black hole and propagate to very large distances – where they bump into the ambient medium,” said Duran, formerly a postdoctoral research associate at Purdue University who is now a faculty member at California State University, Sacramento.

Tchekhovskoy, who has studied these jets for over a decade, said that an effect known as magnetic kink stability, which causes a sudden bend in the direction of some jets, and another effect that triggers a series of shocks within other jets, appear to be the primary mechanisms for energy release. The density of the ambient medium that the jets encounter serves as the key trigger for each type of release mechanism.

For a long time, we have speculated that shocks and instabilities trigger the spectacular light displays from jets. Now these ideas and models can be cast on a much firmer theoretical ground,” said Giannios, assistant professor of physics and astronomy at Purdue.

The length and intensity of the jets can illuminate the properties of their associated black holes, such as their age and size and whether they are actively “feeding” on surrounding matter. The longest jets extend for millions of light years into surrounding space.

When we look at black holes, the first things we notice are the central streaks of these jets. You can make images of these streaks and measure their lengths, widths, and speeds to get information from the very center of the black hole,” Tchekhovskoy noted. “Black holes tend to eat in binges of tens and hundreds of millions of years. These jets are like the ‘burps’ of black holes – they are determined by the black holes’ diet and frequency of feeding.”

While nothing – not even light – can escape a black hole’s interior, the jets somehow manage to draw their energy from the black hole. The jets are driven by a sort of accounting trick, he explained, like writing a check for a negative amount and having money appear in your account. In the black hole’s case, it’s the laws of physics rather than a banking loophole that allow black holes to spew energy and matter even as they suck in surrounding matter.

The incredible friction and heating of gases spiraling in toward the black hole cause extreme temperatures and compression in magnetic fields, resulting in an energetic backlash and an outflow of radiation that escapes the black hole’s strong pull.

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