Measuring a black hole's spin rate

Supermassive black holes -- incredibly dense objects thought to be at the center of spiral galaxies, such as this one depicted in an artist's illustration -- have a mass millions to billions times the mass of our sun.

At the center is an accretion disk, created by matter being pulled toward the center of the galaxy, attracted by gravity. At the core is a region of compact, high energy X-ray radiation thought to originate from the base of jets of the outflowing energetic particles, depicted in blue.

This high energy X-radiation produced by iron atoms lights up the disk, and now data observed from spiral galaxy NGC 1365 in July 2012 by the European Space Agency’s XMM-Newton and NASA's NuSTAR space telescopes has enabled astronomers to see just how speedily matter is swirling in the inner region of the disk, thus accurately confirming the rate of spin for black holes for the first time.
Photo by: NASA/JPL-Caltech

Three types of spin

This chart illustrates the basic method for determining the spin rates of black holes. The three artist's concepts represent the different types of spin: retrograde rotation, where the disk of matter falling onto the hole, called an accretion disk, moves in the opposite direction of the black hole; no spin; and prograde rotation, where the disk spins in the same direction as the black hole.

By breaking the X-ray light up into a spectrum of different colors, scientists assess how close the inner edge of an accretion disk comes to a black hole, which in turn determines the speed of the spin.

To the right of each rotation, we see the resulting spectra for the three spin scenarios. The sharp peak in each chart is X-ray radiation from iron atoms circulating in the accretion disk. If the accretion disk is close to the black hole, as is the bottom row, the X-ray colors from the iron will be spread out by the immense gravitational pull of the black hole. Because the distance depends on the black hole's spin, the spin rate can then be determined.
Photo by: NASA/JPL-Caltech

Two models of black hole spin

The "rotation" model shown at the top of this artist's illustration held that the iron feature was being spread out by distorting effects caused by the immense gravity of the black hole. If this model were correct, then the amount of distortion seen in the iron feature should reveal the spin rate of the black hole.

The alternate model held that obscuring clouds lying near the black hole were making the iron line appear artificially distorted. If this model were correct, the data could not be used to measure black hole spin.

With NuSTAR's data, the alternate "obscuring cloud" theory has been ruled out. High-energy X-ray data -- shown at top as a green bump to the right of the peak -- revealed that features in the X-ray spectrum are in fact coming from the accretion disk and not distorted by the obscuring clouds.

These space observatories were able to make the first conclusive measurement of a black hole's spin rate and confirm that the "gravitational distortion" model is indeed accurate.
Photo by: NASA/JPL-Caltech

X-ray telescopes and electromagnetic spectrum

The NuSTAR and the European Space Agency's XMM-Newton telescopes complement each other by seeing different colors of X-ray light. XMM-Newton sees X-rays with energies between 0.1 and 10 kiloelectron volts (keV), the "red" part of the spectrum, while NuSTAR sees the highest-energy, or "bluest," X-ray light, with energies between 3 and 70 keV.
Photo by: NASA/JPL-Caltech

Isolating X-ray emissions

The ultraviolet-light-monitoring camera aboard ESA's XMM-Newton telescope captured this view of galaxy NGC1365. Previous observations of NGC1365 have detected so many high-energy X-rays being emitted from the galaxy itself and other background sources that accurate data from the black hole was difficult to isolate.

This graphic shows previous X-ray accuracy from the European Space Agency's INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL), and NASA's Swift observatory, but now the NuSTAR X-ray observatory is able to isolate those emissions coming solely from the black hole, allowing for a far more accurate depiction of the X-ray data.
Photo by: ESA

Two theoretical models of X-ray emissions

The solid lines show the two theoretical models explaining low-energy X-ray emissions seen previously from the spiral galaxy NGC 1365 by the XMM-Newton telescope.

The red line explains the emission with a model where clouds of dust and gas partially obscures the X-ray light, and the green line represents a model in which the emission is reflected off the inner edge of the accretion disk, very close to the black hole.

The blue circles show the latest measurements from XMM-Newton, and the yellow circles show the data from NuSTAR. While both models fit the XMM-Newton data equally well, only the disk reflection model fits the NuSTAR data.

The results show that the iron feature, the sharp peak at left, is being affected by the black hole's immense gravity and not intervening clouds. The degree to which the iron feature is spread out reveals the spin rate of the black hole.
Photo by: NASA/JPL-Caltech/ESA/CfA/INAF


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