Two models of black hole spin: Scientists measure the spin rates of supermassive black holes by spreading the X-ray
light into different colors. The light comes from accretion disks that swirl around black
holes, as shown in both of the artist's concepts. They use X-ray space telescopes to
study these colors, and, in particular, look for a "fingerprint" of iron -- the peak shown
in both graphs, or spectra -- to see how sharp it is. Prior to observations with NASA's
Spectroscopic Telescope Array, or NuSTAR, and the European Space Agency's XMM-
Newton telescope, there were two competing models to explain why this peak might not
appear to be sharp.
The "rotation" model shown at top 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.