A new Renaissance Computing Institute (RENCI) animation created from a mathematical model shows a black hole moving supersonically through an interstellar gas cloud. This phenomenon often occurs in multiple star systems, where a companion star provides the gas cloud. The gravity of the black hole pulls the gas inward. Early on in the process, a wake forms behind the black hole, much like the wake behind a motorboat. Unlike a motorboat wake, it begins to move back and forth after a while until it whips all the way around the black hole, forming an accretion disk of gas falling into the hole.
What has mystified scientists since 1988 is the seemingly erratic rotation of accretion disks in the computer-generated models. In some cases the model shows the disk rotating for a time in one direction, then suddenly switching directions. The disk’s spin may remain stable for a time and then abruptly reverse direction again. The reversal may repeat several times.
Astrophysicists refer to this accretion disk about face as “flip-flop instability” and have debated its possible causes for years. Some suggest the phenomenon doesn’t actually occur but is a flaw in the model itself. Yet flip-flop instability has shown up in numerous different studies, leading some scientists to suggest it is the cause of stellar flares and bursts of energy that haven’t otherwise been explained.
North Carolina State University researchers John Blondin and T. Chris Pope generated the data used to construct this visualization. Their computer simulations exploit the power of high-performance computers available through the National Science Foundation’s TeraGrid to explore flip-flop instability at an unprecedented level of detail and scientific sophistication. In a paper published June 30 in the Astrophysical Journal, they conclude that the flip-flop instability is real and not an anomaly of computer models.
In the animation, created by Steve Chall of the RENCI’s North Carolina State Engagement Center, the gas cloud swirls around the black hole at the center, creating an accretion disk around the black hole. At first, the disk spins counterclockwise and then very rapidly reverses to a clockwise direction. The background colors in the animation represent pressure, from green for low through blue, violet, red and finally pale yellow for the highest pressure. Spheres emitted from 10 equally spaced sources upstream from the black hole (far right) show the velocities of representative particles in the gas cloud. Pale yellow spheres exhibit the least velocity, through red, violet and up to blue for the fastest-moving particles.
Credits: Numerical simulation: Dr. John Blondin and T. Chris Pope, department of physics, North Carolina State University. Visualization: Steve Chall, Renaissance Computing Institute NC State. This research was supported in part by a grant from the National Science Foundation, by an NC State Undergraduate Research Award to T. Chris Pope, and by computing resources at the Texas Advanced Computing Center.