Professor’s New Research Shines Light on Black Hole Accretion Discs

New research by College of Charleston physics and astronomy professor Chris Fragile has changed the way scientists look at black holes.

Chris Fragile

Back holes, the collapsed remnants of stars at least eight times more massive than the sun, are some of the most exotic and powerful objects in the universe. Matter falling into them, in a process called accretion, heats up to tremendous temperatures under the strong gravitational influence of the black hole. Some of the energy released in this process is radiated away, primarily in the form of X-rays, before the matter completes its plunge through the event horizon. The X-rays emitted from the infalling matter carry information about the black hole and its surrounding environment, making their collection and measurement a primary mission of NASA and other space agencies for more than four decades.

The X-ray radiation coming from most accreting black holes can roughly be divided into two components, one called the “soft” component (associated with lower energies) and another called the “hard” component (associated with higher energies). One notion for why there are two X-ray components is that the accretion flow itself might be divided into two distinct segments, a relatively cool, thin disk, responsible for the soft X-rays, and a much hotter, thicker corona, responsible for the hard X-rays.

In the 1980s, astronomers discovered that the X-rays, particularly the ones coming from the corona, flicker at certain frequencies, a phenomenon known as quasi-periodic oscillation (QPO). One proposed explanation for the flickering is that the corona wobbles around, like a spinning top, due to something called Lense-Thirring precession. This idea was first proposed about 15 years ago. One problem with this picture, though, was that the frequency of the precession seemed to be too high to match the observed QPOs. However, those earlier predictions only considered the precession of an isolated corona, without an accompanying thin disk. Recent, state-of-the-art computer simulations that include both the corona and the thin disk have demonstrated that the presence of the disk significantly slows down the precession, relieving much of the tension between this model and observations.

“The study of X-ray variability around accreting black holes has long held out the promise to help us better understand that environment; these results hopefully get us one step closer to realizing that promise,“says Fragile.

One interesting feature of this new work is that the precession only happens for accretion flows that are tilted, or misaligned, with the spin axis of the black hole. The magnitude of this tilt may tell astronomers something about how black hole systems form and evolve.

Along with Fragile, the study work was carried out by Deepika Bollimpalli at the Max-Planck-Institut fur Astrophysik in Garching, Germany and Wlodek Kluzniak of the Nicolaus Copernicus Astronomical Center in Warsaw, Poland. The simulations used resources at the College of Charleston, the Texas Advanced Computing Center, Prometheus supercomputing cluster of PL-Grid infrastructure in Poland, and MPCDF clusters in Germany.

The results are published in the Monthly Notices of the Royal Astronomical Society.