Black holes are a never-ending source of astronomical mysteries, but thanks to determined researchers and technological advances, we are finally unlocking their secrets. One curious enigma is the almost regular flickering of the material that surrounds a black hole.
As matter falls into a black hole, it heats up to millions of degrees and starts emitting X-rays. But surprisingly, these X-rays do not remain constant, they dim and brighten at shorter and shorter intervals until the flickering stops.
Using data from ESA and NASA, a group of international astronomers has proven that the flickering is actually caused by rotational frame dragging, a curious effect predicted by Einstein’s general relativity. Their findings are published in the Monthly Notices of the Royal Astronomical Society.
A massive, rotating object will drag space-time along the direction of rotation. The closer you are, the faster your clock will tick.
"It is a bit like twisting a spoon in honey. Imagine that the honey is space and anything embedded in the honey will be 'dragged' around by the twisting spoon," explains lead author Adam Ingram of the University of Amsterdam in a statement. "In reality, this means that anything orbiting a spinning object will have its motion affected."
The bigger the object, the stronger the time-dragging effect, so the team used ESA’s XMM-Newton observatory and NASA’s NuSTAR to follow the stellar mass black hole H 1743-322.
The researchers discovered that X-ray flickering comes from the difference in rotation between the inner and outer disk of material. "We have spent a lot of time trying to find smoking gun evidence for this behavior," said Ingram.
Astronomers actually observe X-rays of a single wavelength (a spectral line), and since the inner disk is being dragged more than the outer region, the line appears to wobble around the expected value in accordance with the prediction of general relativity.
"If you can get to the bottom of the astrophysics, then you can really test the general relativity," added Ingram. This is the first time the rotational frame dragging has been measured in such a strong gravitational field.
The process in not exclusive to huge objects; Earth does it as well. In 2004, NASA launched Gravity Probe B to measure the frame-dragging of our planet. Detailed analysis showed that the effect was real albeit rather small. The spacecraft would take 33 million years to go through a complete rotation.