After less than 9 months in orbit the NuSTAR X-ray telescope, which I discussed in a previous blog post (“A NuSTAR is born“), has produced an important scientific result: it has teamed up with the venerable XMM-Newton to make the first definitive measurement of the spin rate of a black hole.
It’s not easy to measure the spin of a black hole. The key to the measurement is the fact that a rotating accretion disk of gas forms around a black hole, and the gas in the disk gets extremely hot and emits X-rays as it spirals around. However, the disk can get closer to a black hole if the hole is spinning, and thus the X-ray emissions are more strongly affected by the gravity of a spinning black hole than by a non-spinning black hole. Thus if you measure the gravitational redshift in the X-ray emission from an accretion disk, it should be possible to tell whether the black hole is spinning.
Previous measurements from XMM-Newton on supermassive black holes have suggested that those black holes it investigated did indeed have a high spin rate. However, the XMM-Newton results were not conclusive. XMM-Newton measured the X-ray spectrum in the 0.5-10 keV range and at these energies there is another possible explanation for the observations: it’s possible that absorbing layers of surrounding gas clouds mimic the spectrum that would come from a rapidly spinning black hole.
A paper in today’s Nature (“A rapidly spinning supermassive black hole at the centre of NGC 1365” by Guido Risaliti and co-workers) describes how data from NuSTAR and XMM-Newton have been combined to measure the spin rate of the supermassive black hole powering the active galactic nucleus of NGC 1365. Using NuSTAR, Risaliti and his team were able to take a spectrum of photons with energies in the range 3-80 keV. At these extremely high energies the signal is clean, and the data allow a direct comparison between the two possibilities: either a thick layer of gas blankets the accretion or else the black hole is spinning rapidly. It turns out that the “gas-absorbtion” explanation doesn’t work, at least for NGC 1365: for this to be the explanation of the observed spectrum the active galactic nucleus would have to be so luminous that radiation pressure would blow it to smithereens.
It turns out that the supermassive black hole at the centre of NGC 1365 is rotating with at least 84% of its maximum permitted value. And why should we care? Well, astronomers would really like to know how these supermassive black holes evolved and how they affected the evolution of their parent galaxies: did the black holes become supermassive by a gradual process of eating randomly moving clouds of gas and matter, or did they grow by gorging in just a few, gigantic events? If the former is the case, the black hole would rotate slowly; if the latter is the case, the black hole would rotate quickly. In NGC 1365, at least, it seems that the black hole grew to be large in just a few feeding events. If the same sorts of measurements can be made on other galaxies, astronomers will have a much clearer idea of how supermassive black holes and their parent galaxies evolved.