Black holes got their name because their gravity is so strong even light cannot escape it. However, we have been able to detect their presence because beyond the event horizon light doesn't merely leak out, it can shine with astonishing brilliance as material falling inwards is heated to immense temperatures. Theory suggests some of this light, should be twisted around the black hole before it escapes, and now for the first time, this has been seen in action.
In the constellation Normae, a black hole with a mass nine times the Sun's is orbited by a star a little smaller than the Sun. Together the pair is known as XTE J1550-564. The black hole is feeding on its companion, creating an accretion disk that emits X-rays so bright it is known as a microquasar.
Being a relatively modest 14,000 light-years away, the pair has been studied intently, including by NASA's Rossi X-ray Timing Explorer mission, which ended eight years ago. Now a team of scientists has found something in the archival data from 20-year-old outbursts that had not been noticed before.
“We observed light coming from very close to the black hole that is trying to escape, but instead is pulled right back by the black hole like a boomerang," said Caltech's Dr Riley Connors in a statement. "This is something that was predicted in the 1970s, but hadn't been shown until now."
During the outburst the radiation undergoes a shift along the spectrum with time, moving between what astronomers call soft and hard states. These changes provide observers with an opportunity to study the way light from the accretion disk is affected by gravity in a way that would not be possible if it was shining more constantly.
Astronomers were confident in the broad principles of how light would behave in such a strong gravitational field, but less so in estimates of the proportion of radiation that would bend back in this way. In the Astronomical Journal Connors and co-authors report around 5 percent of the X-rays emitted by the inner accretion disk undergo this extreme bending, consistent with theoretical models.
Besides confirming things we already suspected, the team believes their discovery can be used to investigate black holes' spin. "Since black holes can potentially spin very fast, they not only bend the light but twist it,” Connors said. It's hoped observations of XTE J1550-564 and objects like it will help us understand how fast black holes spin and the forces that determine this.