Up to 10 keV, X-ray spectra produced by accreting supermassive black holes bare the signatures of two nearly indistiguishable models that describe very different astrophysical scenarios. On one side, the observed spectral curvature maybe the result of high energy radiation emitted by accretion mechanisms and modified by the strong gravitational field of the black hole (Gravitational Distortion, green line in the figure). On the other side, the same spectral shape can be produced if clouds of dust and gas partially block the X-ray emission from the compact object (Gas Obscuration, red line). This second illustration shows the two models in more detail. Until now, telling the two scenarios apart has been impossible.
For roughly 36 hours in July 2012, ESA's XMM-Newton and NASA's NuSTAR – the Nuclear Spectroscopic Telescope Array – simultaneously observed the spiral galaxy NGC 1365. XMM-Newton captured the lower energy X-rays, NuSTAR the higher energy data.
The combined data proved to be key to unlocking the puzzle. A spinning black hole model makes a clear prediction for the ratio of high-energy to low-energy X-rays. The same is true for an absorbing cloud of gas.
But importantly, the predictions are different and the new data agree only with a rapidly spinning black hole. This suggests that the galaxy has grown steadily with time, with material streaming uniformly into the central black hole.
Measuring black hole spins also provides a new way to test general relativity. Published in 1915, general relativity is Albert Einstein's description of gravity. It predicts effects that are most easily seen in extremely strong gravitational fields, such as those found near black holes, and NGC 1365's black hole is spinning almost as fast as Einstein's theory of gravity will allow.
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