General relativity caught in action around black hole

GENEVA — An effect of general relativity that is barely measurable on Earth has been spotted in full force around a black hole.

Physicists detected the signature of a black hole twisting the fabric of spacetime around it. The discovery offers the best evidence yet of this relativity-driven twisting effect, known as frame dragging, around a black hole where it is most powerful. The research was reported December 16 at the Texas Symposium on Relativistic Astrophysics.

Researchers captured the extreme frame dragging by analyzing X-rays emanating from a disk of star debris swirling around a black hole about 28,000 light-years away in the Milky Way. The data suggest that the disk’s matter is on a wild ride as the spacetime it occupies gets yanked and warped by the spinning black hole.

Albert Einstein’s century-old general theory of relativity describes gravity in terms of massive objects deforming the surrounding spacetime. For example, Earth creates a dent in spacetime much like a bowling ball would on a rubber sheet. The concept of frame dragging is less intuitive: It stipulates that if the ball were spun, it would drag the sheet along with it.

Physicists with the Gravity Probe B project measured Earth-induced frame dragging using gyroscopes inside a satellite (SN: 12/26/15, p. 7). If the rules of relativity had not applied, the axis of each gyroscope’s spin would have pointed in the same direction forever. But the researchers found that the axes deviated by about a hundred-thousandth of a degree per year due to Earth’s rotation. The experiment required extreme sensitivity to capture such a subtle effect.

But frame dragging should be anything but subtle around a black hole, which packs an immense mass within a small volume. While scientists can’t put a satellite into orbit around a black hole, they can study the motion of stuff circling it. Adam Ingram, an astrophysicist at the University of Amsterdam, and colleagues zeroed in on H1743-322, a black hole that is stripping matter from an unlucky star. The disk of material orbits on a plane that is not quite perpendicular to the black hole’s axis of spin.

Using data from the XMM-Newton space telescope, the researchers analyzed X-rays emitted by iron ions embedded in the swirling disk of stellar material. These ions emit X-rays at a telltale frequency, but that frequency grows and shrinks slightly depending on the direction the ions are moving in relation to the observer. Ingram and colleagues studied how the frequency of the iron-emitted X-rays fluctuated over time to chart the path of material in the disk.

Based on the pattern of frequency shifts, the researchers concluded that, in addition to orbiting the black hole, the disk is also wobbling: As the black hole spins, it tugs on the surrounding spacetime and drags the disk with it. The disk’s innermost material experiences a frame-dragging effect that’s about 100 trillion times as strong as the effect experienced by the Earth-orbiting gyroscopes, Ingram reported. The axis of a gyroscope in black hole orbit would drift roughly 90 degrees each second.

“This result is very big,” says Eugenio Bottacini, an astrophysicist at Stanford University who attended the presentation where the results were announced. But he wants to see details of the analysis in Ingram’s upcoming paper, which is under review for publication.

In his presentation, Ingram said the research illustrates how scientists can use iron-emitted X-rays as a scanner to view black hole accretion disks from different angles as the material orbits and wobbles. Additional studies could enable scientists to further test Einstein’s seminal theory and better understand the conditions facing matter caught in a black hole’s gravitational clutches.