Two space telescopes, NASA’s Nuclear Spectroscopic Telescope Array (NuStar) and ESA’s XMM-Newton, were used together to measure for the first time the spin rate of a black hole. The object of the study is the supermassive black hole at the center of the galaxy NGC 1365, which according to findings spins at near the speed of light.
NGC 1365 is a barred spiral galaxy, which has a central bulge of stars with two extensions which together resemble a bar that passes through the core. It’s located about 56 million light years from Earth and the supermassive black hole at its center has a mass of about two million times the Sun
The enormous energies produced when matter falls in the extremely powerful gravitational well of a supermassive black hole determine the X-ray emission, which can be detected to understand its characteristics, including its spin rate. In particular, it’s useful to observe the X-rays emitted just outside its event horizon, the border around the black hole beyond which even light can’t escape its force of gravity.
The problem is that around the black hole there are clouds of dust which may shield the X-ray emissions confusing the results of the measurements. The NuStar space telescope, launched in June 2012, allows measurement of high-energy X-rays in details while the XMM-Newton allows the observation of X-rays at lower energies. The simultaneous use of the two space observatories allowed scientists to study the supermassive black hole penetrating more deeply the region around it.
The data obtained by observing the high-energy X-rays revealed that those emissions weren’t bent by the clouds of dust but by the strong black hole gravity. This proves that it’s possible to measure the spin rate of supermassive black holes, that in the case of the one in the galaxy NGC 1365 is close to that of light.
The study carried out by the team led by Guido Risaliti of the Harvard-Smithsonian Center for Astrophysics in Cambridge and the Astrophysical Observatory of Arcetri, in Italy, allows scientists to understand the past of the galaxies from the characteristics of the supermassive blacks holes at their center.
According to Einstein’s theory of general relativity, the higher the spin rate of a black hole, the greater the proximity of the accretion disk formed by the matter that is drawn towards it. The closer this disk, the more the black hole gravity will bend the X-rays.
The iron atoms emit strong X-rays “signatures” with specific energies therefore they’re the most observed for measurements. In the case of the supermassive black hole in the galaxy NGC 1365, the XMM-Newton space telescope revealed that the X-rays emitted by iron were bent while NuStar showed that the distortion came from the black hole gravity because the iron was so close to the black hole that only gravity could cause such effect.
This new research also helps scientists understand the evolution of galaxies. In the case of NGC 1365, a rapidly spinning black hole suggests a steady growth with material that is finished uniformly into the black hole. However, it’s possible that two galaxies and the blacks holes at their center have merged to form the present NGC 1365.
This type of research will go on to discover more about NGC 1365 and other galaxies to get a better idea of the evolution of supermassive blacks holes and the galaxies that contain them, therefore of the whole universe.