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Bubble Shock Trains

Published: 04-10-2006

Black holes are pretty strange, sucking in not only nearby matter but also the space around it. These cosmic vacuum cleaners are powered by thin, gaseous accretion disks in orbit around them. Something drives the orbiting gas to spiral in toward the black hole, where all trace of it disappears forever into the singularity. One of the exciting challenges in astrophysics is to figure out the physics driving this process, which keeps black holes growing for billions of years after they're formed.

For the past five years, Fellow Mitch Begelman and colleagues from the University of California at Santa Barbara, Princeton University, and NASA's Jet Propulsion Laboratory have been studying the effects of photon bubbles on the evolution of black hole accretion disks. Photon bubbles are cavities of X-rays that form in the sea of electrically charged gas (plasma) found in accretion disks with strong magnetic fields. These exotic objects are also found in the intensely magnetic polar regions of neutron stars.

The researchers have discovered that photon bubbles grow fastest along magnetic field lines that are nearly vertical. Spurred by gravitational acceleration, they also grow faster in accretion disks surrounding large black holes. In some accretion disks, photon bubbles can spontaneously evolve into bubble wave trains traveling toward the outer edge of the disk. These bubbles, which form in less dense areas of the gaseous disk, are often buoyant. As they rise up through the disk, their behavior resembles convection patterns found in the Sun. These are the photon bubbles found in neutron stars.

Photon bubbles are also formed via the interplay between pressure from radiation and pressure from dense gases in an accretion disk. These bubbles, which behave like sound waves, develop rapidly. They can evolve into gas-pressure-driven shock wave trains that travel from the middle of the disk toward both the upper and lower surfaces. The shock waves (in the computer simulation on the right) are formed when radiation slams gas into the shock front as gravity squeezes the gas behind the front. The density between waves of the shock train drops, allowing radiation to reaccelerate the gas. The (blue) low-density regions in the shock wave train allow radiation (and heat) to escape from the accretion disk.

Begelman and his colleagues posit that photon bubbles may be critical for cooling accretion disks dominated by high-energy radiation. Such cooling is vital to the long-term stability of the disk and to the black hole's power supply. To better understand how photon bubble evolution relates to overall accretion disk dynamics, Begelman and his colleagues are currently working to determine how likely photon bubbles are to survive turbulence generated by other disk processes.

Two recent articles authored by Begelman and his collaborators on photon bubbles appeared in the May 1, 2005, and January 10, 2006, issues of The Astrophysical Journal.   - Julie Phillips


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