Tuesday, March 5, 2013

Black Hole Spin

Astronomers recently combined data from the Nuclear Spectroscopic Telescope Array (NuSTAR) with the European Space Agency’s XMM-Newton X-ray satellites to observe the spin of a black hole. This is the first time anyone has been able to do this. In the center of galaxy NGC 1365 lies a supermassive black hole with a mass about two million times that of our Sun. Astronomers measured it to be spinning pretty much as fast as Einstein’s theory of relativity will allow.

These findings have been published in the latest edition of “Nature” and settle the debate on similar measurements of other black holes. The results are essential to black hole science and will lead to a better understanding of black holes and their host galaxies.

Einstein’s theory of general relativity describes how gravity bends space-time and the light that passes through it. A black hole’s gravity is so strong that when it spins, it actually drags the space around it along. The surrounding material collects and forms an accretion disk, heats up due to friction and emits x-rays. Matter swirling in can be traced by these x-rays and the radiation seen is warped and distorted by particle motions and gravity.

NGC 1365’s x-rays were measured from the galaxy’s center and astronomers were able to locate the inner edge of the accretion disk – also thought of as “the point of no return”. The location of this point is determined by the black hole’s spin. As we know, black holes distort space so the material can get incredibly close before actually being sucked in. Why is this important? Black holes are defined by two things: spin, and mass. These two numbers can tell scientists almost everything they need to know about a specific black hole.

NuSTAR is designed to detect highest-energy x-ray light in great detail. It compliments telescopes that observe lower energy x-ray light such as Chandra or the XMM-Newton. In the past black hole measurements were not concise due to gas clouds obscuring results. When used in combination, NuSTAR and XMM-Newton are able to “see” a broader range of x-rays and actually penetrate the clouds surrounding a black hole.

Black holes are surrounded by large disks of material known as accretion disks, which are formed as matter is sucked in. Einstein’s theory of relativity predicts that the faster a black hole spins, the closer the accretion disk is. Also the closer the accretion disk, the more x-ray light is warped by gravity. Astronomers look for the warping effects and study x-ray data emitted by iron circulating in the accretion disk.

XMM-Newton showed the warped light emitted from the iron and NuSTAR proved this was due to gravity. Talk about a telescopic tag team! NuSTAR’s data showed the iron was so close to the black hole that the only explanation for warping was gravity. Once the possibility of obscuring clouds had been ruled out, astronomers used the iron signature distortions to measure spin rate.

NGC 1365’s spin rate was close to maximum which shows scientists it grew through organized accretion and not just randomly. Astronomers will also be able to use the data collected to determine how spin rates change over time.
These two telescopes are currently being used to study other black holes in the same fashion and compare results to those of MGC 1365.

The image seen here is an artist’s illustration of a super-massive black hole (like NGC 1365) surrounded by the accretion disk.

For more reading on accretion disks check out our post:

For further reading on gravity and Einstein’s Theory of General Relativity check out our post:


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