Black holes have always provided a mystery to astronomers. They cannot be observed directly due to their strong gravitational pull which even prevents light from escaping past the event horizon (which is the point of no return). Inside their depths could lay many secrets about the history of our universe. Although black holes can’t be observed directly matter in the accretion disk, the area of debris around the star, which is about to enter the black hole can.
Within the accretion disk are iron atoms, which absorb x-ray radiation produced by gas as it falls into the black hole. Iron atoms re-emit x-ray radiation at a specific frequency, allowing it to then be observed using telescopes. This iron emission line can then be used to determine the spin of the black hole.
The black hole’s spin is like the black hole’s memory; it can be used to tell astronomers about the formation and evolution of black holes. At the centre of every galaxy, including our own, is a black hole. Knowing about the history of these black holes leads to knowledge about their host galaxies, and ultimately gives an insight into the growth of the universe.
A recent experiment published in ‘Nature’ had the aim of determining the spin of the black hole in NGC1365. The experiment was possible due to the collaboration of the NuSTAR and the XMM-neutron telescopes. The NuSTAR telescope was used to prove that gas clouds did not obscure observations. This ensured that scientists could be confident that distortions in the iron emission line, which were observed by the XMM-Newton telescope, were due to the black hole.
Distortions in the iron emission line could then be confidently used to calculate the rate at which the black hole is spinning. The spin observed by the telescopes was large and so suggests black holes did not grow gradually but as a result of a few large absorptions of matter, perhaps from the merging of black holes. This experiment made the first precise black hole spin measurement and removed the uncertainty from other previous results. Although in order to properly begin to analyse spin, more black holes will have to be observed.