Gravitational lensing enabled a team of researchers to measure the spin of five black holes — and one was spinning as fast as is theoretically possible.

Image credits: NASA/CXC/Univ. of Oklahoma/X. Dai et al.

It’s pretty hard to learn new things about something you can’t even see, yet in the new study, that’s exactly what researchers have accomplished: they’ve gauged the spin of five supermassive black holes using a combination of data from the Chandra X-Ray Observatory and chance alignments.

When black holes are spinning, they create whirlpool-like perturbations in the galaxies around them. However, instead of eddy currents, black holes produce bright disks of gas and light which glow brightly in x-ray light. But that isn’t enough.

The process was also facilitated by something called microlensing — a type of phenomenon produced by the gravitational lens effect. Essentially, the phenomenon describes how the distribution of extremely heavy matter between a light source and an observer is capable of bending the light as it travels. The light is distorted in a way that’s similar to a magnifying lens, but it requires a very specific alignment.

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A light source passes behind a gravitational lens (point mass placed in the center of the image). The aqua circle is the light source as it would be seen if there was no lens, white spots are the multiple images of the source. Image via Wikipedia.

In this case, Chandra allowed the gathering of sharp images, in which the black holes were distinguishable from the multiple images produced by the lensing. From there, microlensing was used to detect the spin. A smaller emitting region corresponds to a tighter orbit and a faster spin speed — in other words, the smaller the region emitting X-rays, the faster the black hole spins.

Researchers were surprised to see that one particular hole was spinning at 670 million miles per hour which corresponds to 70% of the light speed. Four other black holes were spinning about half this speed, and for one, a reliable measurement couldn’t be taken because the conditions were not exactly right.

So how can the black hole spin so rapidly? Astronomers believe that it accumulated matter over billions of years, sucking in its initial accretion disk, as well as any matter in the area. If this matter (and particularly the accretion disk) was acquired in the direction of the spin, it’s like a carousel of constantly accelerating matter and the black hole continued to pick up speed.

Of course, observations like this one are rare because they require a very specific alignment, as well as very distant (and powerful) black holes — in this case, located from 8.8 billion to 10.9 billion light-years from Earth. They also require a lot of time from Chandra, as the photos were taken with total exposure times ranging between 1.7 and 5.4 days.