A recent article in Nature describes how a supermassive black hole’s spin has been measured via the gravitational lens of a foreground galaxy, that fortuitously lies along the same line-of-sight. The new measurements have enabled astronomers to find that a supermassive black hole powering a distant quasar has grown through coherent, rather than random, episodes of mass accretion (see Section 17.3, p. 458-459). The following text is a digest of Guido Risalti’s summary in Nature‘s ‘News and views’ section, March 13, 2014.
Supermassive black holes are simple systems. They are characterized by just two quantities, their mass and their spin. Whereas the total amount of accretion and any mergers that a supermassive black hole undergoes are encoded in its mass, how this mass was assembled is encoded in its spin. A few ordered accretion events or mergers of large black holes produce high spins, and short, random accretion processes produce low spins. Measuring these spins is therefore a major goal of extragalactic astronomy: the spins of supermassive black holes hold a key to understanding the evolution of their host galaxies.
But how can we measure the spins? According to Einstein’s general theory of relativity, a black hole’s gravitational field twists space-time around it. Such twisting depends on the black hole’s spin, so measuring the twisting allows the spin to be estimated. The signature of space-time distortion is imprinted on the emission of radiation from regions close to the black hole’s event horizon – the surface beyond which no radiation can escape. The best way to perform such a measurement is to observe X-rays reflected by the disk.
In their study, R. C. Reis and colleagues break new ground by obtaining a spin measurement of a quasar at a distance of more than 6 billion light years from Earth, from a time when the Universe was about half its current age. This remarkable result was possible owing to the exceptional nature of the observed source – a quadruply imaged, gravitationally lensed quasar.
The light from the distant quasar is both magnified and split into four different images by the gravitational field of a foreground elliptical galaxy (the lens) that, by chance, is on the line of sight of the quasar. For this reason, the authors could analyse four ‘copies’ of the X-ray spectrum of the quasar, each with an intensity significantly magnified by the lens. The resulting X-ray spectra have a quality that matches the best that has been obtained for nearby sources, and allowed a robust measurement of the black hole’s spin. As it turns out, the spin is large (close to the highest possible value that theory predicts), suggesting that the black hole acquired its mass through coherent phases of mass accretion.