Mathematical Geophysics News

New Proof That Accretion Disks Align with Their Black Holes

In the most detailed and highest-resolution black hole simulation to date, an international team of researchers showed the Bardeen-Petterson effect for the first time.

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In 1975, physicist James Bardeen and astrophysicist Jacobus Petterson theorized the existence of a black hole phenomenon that researchers have since been scrambling to show.

In a study published in the July issue of Monthly Notices of the Royal Astronomical Society, researchers announced that they finally demonstrated Bardeen-Petterson alignment, in which a spinning black hole causes the inner portion of a tilted accretion disk to align with the black hole’s equatorial plane. Finding this effect could change our understanding of how black holes grow and how their presence affects galaxies, according to Sasha Tchekhovskoy, a computational astrophysicist at Northwestern University and colead author of the study.

Colorful, detailed graph showing Bardeen-Petterson alignment
In this image, the inner region of the accretion disk (red) aligns with the equatorial plane of the black hole, while the outer disk tilts away. The inner disk (where the black curve dips) is horizontal due to Bardeen-Petterson alignment. Credit: Sasha Tchekhovskoy/Northwestern University and Matthew Liska/University of Amsterdam

Powerful Resources Fueled the Simulation

To accomplish the most detailed and highest-resolution black hole simulation to date, Tchekhovskoy and his team used the Blue Waters supercomputer at the University of Illinois at Urbana-Champaign. They also used adaptive mesh refinement, a research method that uses grids that change in response to movements within simulations, and a technique called local adaptive time stepping to bring down the simulation cost by 2 orders of magnitude.

“It’s very difficult to model the [accretion] disks that show this effect” because they are extremely thin, said Tchekhovskoy. Using graphical processing units instead of central processing units (previously used in similar black hole simulations) enabled the team to “simulate the thinnest disks to date,” Tchekhovskoy said.

These thin accretion disks have height-to-radius ratios of 0.03, and Tchekhovskoy says the team was surprised to discover that “all of these interesting effects,” including Bardeen-Petterson alignment, appear at that thickness. The thinnest disks simulated prior to this study were more than 1.5 times thicker.

Cole Miller, an astrophysicist at the University of Maryland in College Park who wasn’t involved with the new study, said he’s impressed with the level of detail in the simulation.

Unexpected Jets

“A major surprise of this work is the finding of powerful jets, even in our thin disc accretion system,” the researchers wrote in the study.

The finding runs counter to the team’s expectation that magnetic fields would rip through the thin accretion disks rather than producing jets, Tchekhovskoy said. Exploring this finding in future work could provide insights into a different phenomenon, he added: why only about 10% of bright, supermassive black holes produce these jets.

Putting Bardeen-Petterson in Context

Miller noted that some initial coverage of the new study incorrectly identified John Bardeen, James’s father and a two-time winner of the Nobel Prize in Physics, as one of the two people after which the Bardeen-Petterson effect is named.

Besides theorizing the Bardeen-Petterson alignment, James Bardeen, now professor emeritus at the University of Washington in Seattle, is also known for formulating (with Stephen Hawking and Brandon Carter) the laws of black hole mechanics. Jacobus Petterson, who died in 1996, was “best known in the astronomical community for his analysis of X-ray binary systems,” according to an obituary written for the American Astronomical Society.

“This groundbreaking discovery of Bardeen-Petterson alignment brings closure to a problem that has haunted the astrophysics community for more than four decades,” Tchekhovskoy said in a statement.

“It’s an interesting paper and definitely takes things a step or two beyond previous work,” according to Julian Krolik, an astrophysicist at the University of California, Berkley, who wasn’t involved with the study.

However, Krolik disagrees about the overall importance of the paper. “It’s not ‘ground-breaking,’ nor does it provide ‘closure.’ ‘Closure’ in this field would mean identification of all the principal mechanisms regulating where the steady-state orientation transition takes place, construction of a method to predict (given relevant disk parameters) the location of this transition, and definition of the boundaries in parameter space separating where alignment is successful and where it isn’t,” Krolik wrote in an email to Eos.

There is one major question that Krolik said the researchers leave unanswered: why the accretion disk alignment “extends to only a short distance from the black hole, stopping far short of where they expected it to reach.”

—Rachel Crowell (@writesRCrowell), Science Journalist

Citation: Crowell, R. (2019), New proof that accretion disks align with their black holes, Eos, 100, https://doi.org/10.1029/2019EO127561. Published on 10 July 2019.
Text © 2019. The authors. CC BY-NC-ND 3.0
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