Phil Armitage

JILA Fellow Phil Armitage moves on

Steven Burrows — Tue, 25 Jun 2019 17:22:00 +0000

JILA Fellow Phil Armitage moves on Steven Burrows Tue, 06/25/2019 - 11:22

Rebecca Jacobson / JILA Science Communicator

JILA Fellow Phil Armitage, second from left, is leaving CU. Image Credit: Molly Alvine/JILA

JILAns bid "bon voyage" to Fellow Phil Armitage on Friday.

While at JILA, Armitage researched theoretical and computational astrophysics, focusing on planet formation and the physics of black holes. His recent work includes the development of a new model for high-energy accretion based on strongly magnetized disks and analytic and simulation studies of planetesimal formation.

Armitage will be moving on to Stony Brook University. Best of luck, Phil! We wish you luck in your next endeavor.

JILA Fellow Phil Armitage is moving on from the University of Colorado.

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Dancing with the Stars

Steven Burrows — Tue, 22 Nov 2016 19:23:21 +0000

Dancing with the Stars Steven Burrows Tue, 11/22/2016 - 12:23

Julie Phillips / Science Communicator

Computer simulation of a tidal disruption event involving a pair of supermassive black holes in the center of a recently merged galaxy. The flow patterns create a distinctive signal of the presence of a pair of closely orbiting black holes. Image credit: Eric R. Coughlin, JILA

Galaxy mergers routinely occur in our Universe. And, when they take place, it takes years for the supermassive black holes at their centers to merge into a new, bigger supermassive black hole. However, a very interesting thing can happen when two black holes get close enough to orbit each other every 3–4 months, something that happens just before the two black holes begin their final desperate plunge into each other. And, according to former JILA graduate student Eric Coughlin and his colleagues, if one of the black holes happens to tidally disrupt an errant star, the process will send out a signal that will allow Earthlings to “see” which galaxies contain these pairs of black holes.

“If two black holes happen to be that close together, and a star gets disrupted by one of the black holes, there’s a reasonable probability that the debris stream will actually miss the black hole that disrupted it and hit the second black hole,” said Mitch Begelman, Coughlin’s thesis advisor at JILA. “You get this kind of dance between the two black holes, and of course you get fantastic flow patterns that are just neat.” Begelman added that these flow patterns create a distinctive signal that there are two black holes involved in the tidal disruption of a single star.

Right now, existing space-based telescopes could detect one of these events every few years. However, in 2019 or 2020, the huge Large Synoptic Survey Telescope (LSST) will come online. And, thanks to Coughlin’s new study that tells astronomers what to look for, the LSST should be able to see a handful of the binary black-hole mergers every year among the many galaxies in our Universe.

“It is a notoriously difficult thing to discern the presence of one black hole, and this is a way to find two,” Coughlin explained. “We think binary black-hole systems should be common, considering how we think our own galaxy evolved via multiple galactic collisions.”

Coughlin said that astronomers now have a new probe in tidal disruption events, which are well understood, to learn something about the evolution of galaxies. As part of his research into tidal disruption events and how they can be used to identify pairs of black holes in the center of merging galaxies, Coughlin has created a stunning animation of the process in action.[2]

The researchers responsible for this work include recently minted JILA Ph.D. Coughlin, recent visitor and former research associate Chris Nixon, and Fellows Phil Armitage and Mitch Begelman.

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Black Hole Marvels

Steven Burrows — Thu, 11 Aug 2016 18:40:47 +0000

Black Hole Marvels Steven Burrows Thu, 08/11/2016 - 12:40

Julie Phillips / Science Communicator

Magnetized accretion disks around different black holes. (Top) An accretion disk produced by a weak vertical magnetic field. (Bottom) An accretion disk produced by a strong vertical magnetic field that is strong, but not too strong. (Not shown) A vertical magnetic field that is too strong will fail to magnetize an accretion disk. Image credit: Steven Burrows / JILA

Graduate student Greg Salvesen, JILA Collaborator Jake Simon (Southwest Research Institute), and Fellows Phil Armitage and Mitch Begelman decided they wanted to figure out why swirling disks of gas (accretion disks) around black holes often appear strongly magnetized. They also wanted to figure out the mechanism that allowed this magnetization to persist over time. In the process, they hoped to explain some intriguing observations of super-gale force winds blown off of some black-hole accretion disks. So the researchers created supercomputer simulations that allowed them to come up with some interesting ideas about the behavior of magnetized disks.

What they learned is that if there is a magnetic field that is poking vertically through the black hole’s accretion disk, a wreath-shaped magnetic field will be generated within the accretion disk. And, the stronger the vertical field is, the stronger the magnetic wreath will be inside the gas making up the accretion disk––up to a point.

“What we’ve shown is that to get a very strongly magnetized disk, the wreath-shaped component is dominant,’ said Salvesen. “But you still need that vertical magnetic field, which is subdominant, but very important for the physics that produces and maintains a strong wreath-shaped magnetic field.”

Salvesen explained that there are generally two types of black holes that sometimes have accretion disks: supermassive black holes (at the center of each galaxy) with masses of millions to billions of Suns and stellar-sized black holes with masses of about 5 to 30 Suns that are littered throughout every galaxy. Light coming from these accretion disks helps astrophysicists understand both the properties of black holes and the physics of the disks.

“There have been observations recently that are suggestive of some disks being strongly magnetized,” Salvesen explained. “But the most compelling observations are the fast winds we sometimes see coming from the disks. These winds travel at a few percent of the speed of light and are about 10,000 times more powerful than the strongest hurricane winds on Earth!”

The winds must be launched away from the disks by some kind of pressure. There are three possible mechanisms that could create winds: thermal pressure from hot gas; radiation pressure from light; or pressure from magnetic fields. Salvesen is focusing on the pressure from magnetic fields.

“Radiation and thermal pressure often can’t cut it, because they can’t make a wind powerful enough,” he said. “In these cases, the magnetic fields must be responsible for producing the winds we observe coming off the disk.”

Consequently, Salvesen decided to further explore the formation of magnetic fields in black hole accretion disks. What he discovered is fascinating.

“If you have a disk that is initially not turbulent and then you thread it with a vertical magnetic field, an instability will generate turbulence within the disk,” he said. “Even the very weakest field will cause the disk to become turbulent. But, if that vertical magnetic field is too strong, then this instability won’t happen. The vertical magnetic field has to be just right––strong, but not too strong.”

The vertical field needs to be just strong enough to create a very strong magnetic wreath. The magnetic wreath then governs the behavior (and the physics) of the disk from then on, so long as a vertical magnetic field remains in place. In black-hole accretion disks where such conditions can exist, strong magnetic wreaths may play a role in producing extremely fast winds that can be observed by Earthlings hundreds of light years away.

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