Supermassive black hole’s relativistic spin rate measured for the first time

Two X-ray space observatories have teamed up to make the first measurement of a supermassive black hole’s spin.


An artist’s impression of a supermassive black hole millions to billions times the mass of our Sun buried at the heart of a galaxy. An outflowing jet of energetic particles shoot from the exotic object and is thought to be powered by the spin.

They say that nothing can escape a black hole but it seems that the collaboration of two X-ray space observatories, NASA’s NuSTAR and ESA’s XMM-Newton, has managed to prize away a measurement of the spin rate of one of these supermassive objects for the very first time. The exotic object was found to be pirouetting at a relativistic velocity close to the speed of light.

Weighing in at a mass of two million times that of our Sun, the black hole that grabbed the attention of a team of scientists led by Guido Risaliti of the Harvard-Smithsonian Center for Astrophysics in Massachusetts and the Italian National Institute for Astrophysics, takes pride of place at the heart of the Great Barred Spiral Galaxy NGC 1365 at 56 million light years away in the constellation Fornax.

Spinning at a hectic speed which almost matches the rate allowed by Einstein’s theory of gravity, the properties of this supermassive black hole attempt to resolve a long-lived debate about similar measurements in other black holes leading to a more solid vision of how black holes and galaxies evolve.

And that’s not all; these new observations are also a powerful tool when it comes to testing Einstein’s theory of general relativity which states that gravity bends the space-time fabric of our Universe along with the light that permeates it. “The only way to observe strong effects of gravitational fields on the surrounding space-time is to study the surrounding of black holes, which, by definition, produce the strongest possible fields,” says Risaliti. “Close to the event horizon (i.e the no-return point) of a black hole, space and time are heavily distorted, all new phenomena happen, also depending on the black hole spin, and general relativity can be tested in its full extent.”

These heavy weight black holes are surrounded by a pancake of material known as an accretion disc, made as gravity pulls matter inward. It is thought, according to Einstein’s general theory of relativity, that the closer an accretion disc lies to its black hole, the more gravity will warp any X-rays emanating from iron that circulates within it.

The electromagnetic spectrum, highlighting the X-ray portion.

Without the marriage between high-energy X-ray detector NuSTAR and XMM-Newton, scientists were previously unsure if their black hole target was so enshrouded by clouds of gas, that results were confused. However with the duo’s combined power, a broad range of X-ray energies appeared and they were able to penetrate deeper into the region around the supermassive object. With XMM-Newton uncovering light was being warped, NuSTAR demonstrated that the proximity of iron to the hefty black hole was in fact, causing the warping effects.

“If I could have added one instrument to XMM-Newton, it would have been a telescope like NuSTAR,” admits Norbert Schartel, Project Scientist of the low-energy X-ray XMM-Newton who is currently at the European Space Astronomy Center in Madrid. “The high energy X-rays provided an essential missing puzzle piece for solving this problem.”

With the new data, the team found that X-rays were not warped by the clouds, but instead, by the tremendous gravity of the black hole allowing its spin rate to be determined. This characteristic almost served as a signpost to understanding, not just the black hole’s past, but also that of its host galaxy.

“We believe supermassive black holes are not born so big; in the early Universe they are small seeds, and they grow through accretion of gas and stars, or through mergers with other giant black holes, when their host galaxies collide,” says Risaliti who explains that depending on how a black hole forms, the final spin is influenced by it.

“An ordered, continuous accretion of gas and stars from a galactic disc would add angular momentum to the black hole always in the same direction, thus spinning up,” he tells All About Space. “Instead, a series of many unrelated accretion events from random stars and clouds would add momentum in random directions, sometimes spinning up or sometimes spinning down the black hole.”

So what’s next for Risaliti’s team? “We are repeating the simultaneous XMM-Newton and NuSTAR observations three more times,” he answers. “The comparison of these different observations should provide a further, even stronger confirmation of our results, and of course, provide a more precise measurement of the black hole spin.”

You can follow Gemma on Twitter @Gemma_Lavender

Images courtesy of NASA/JPL-Caltech

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