Astrophysics

Black holes in a spin

The implications of the X-ray emission patterns of galaxies hosting supermassive black holes have been contentious. Data from NASA's NuSTAR telescope seem to resolve the issue — at least for one such galaxy. See Letter p.449

Supermassive black holes, with masses of millions to billions times that of our Sun, are believed to exist at the centre of essentially every galaxy. When these monsters feast upon the gas (and possibly the stars!) within galactic centres, they release enormous quantities of energy, producing a phenomenon called an active galactic nucleus (AGN). Far from being dark and difficult to detect, the black holes in AGNs are the most luminous spectacles in the Universe. On page 449 of this issue, Risaliti et al.1 report observations of the AGN at the centre of the nearby galaxy NGC 1365 (Fig. 1) using the Nuclear Spectroscopic Telescope Array (NuSTAR), a newly deployed X-ray observatory and NASA's latest addition to its fleet of space telescopes. By exploiting NuSTAR's ability to measure the high-energy X-ray spectrum of an AGN with unprecedented accuracy, the authors obtain an unambiguous measurement of the spin rate of this supermassive black hole, finding a spin that is at least 84% of the maximum theoretically allowed value.

Figure 1: Spiral galaxy NGC 1365.
figure1

ESO

Risaliti et al.1 have measured the spin rate of the supermassive black hole that lurks at the centre of NGC 1365, shown here in an optical image obtained with the Very Large Telescope.

Why should we care so much about these supermassive black holes or their spin? To start with, their very presence is a mystery that draws in the curious astrophysicist. It now seems clear that the first black-hole 'seeds' were created just a few hundred million years after the Big Bang, although the process that created them is still not understood. Weighing in at a mere 10,000 solar masses or so, these seeds then gorged upon the gas within the young galaxies and grew rapidly into the behemoths that we see today. Furthermore, our understanding of galaxy formation and evolution is intimately linked to our understanding of supermassive black holes. The energy released by a growing supermassive black hole can be so powerful that it disrupts the normal growth of the host galaxy; in extreme cases, the AGN can terminate all subsequent growth of the galaxy.

Although essentially every detail of this feasting process is uncertain, the spin of a supermassive black hole can help us unravel the mystery of its growth2. If a black hole grew in one (or a small number of) dramatic feeding event(s), it would acquire the angular momentum of the inflowing matter and would be spinning very fast. On the other hand, growth through the infall of small, randomly oriented packets of gas (or even small black holes) would produce black holes that rotate much more slowly3. In this way, the black-hole spin is a 'fossil remnant' of its formation processes.

Black-hole spin — which reveals itself in the twisting of space-time close to the hole's event horizon, beyond which no matter or light can escape — is a difficult quantity to measure. Our handle on spin comes from the fact that a spinning black hole 'draws in' the inner edge of the accretion disk, the flat rotating disk through which gas flows into the black hole. Because the accretion disk can get closer to the black hole when the black hole is spinning, the disk's emissions are more strongly affected by the gravity of the black hole, producing an enhancement in the gravitationally induced redshifting of the disk's emission spectrum4,5. Through a detailed spectral analysis of the X-ray emission from the accretion disk, we can model these effects and determine the black-hole spin.

Black-hole spin measurements using this technique have been conducted6,7 for several years using the 0.5–10-kiloelectronvolt part of the X-ray spectrum accessible with the sensitive spectrographs on previous X-ray missions such as the Chandra X-ray Observatory, XMM-Newton and Suzaku. However, these measurements remained somewhat controversial because of the existence of an alternative interpretation of this part of the X-ray spectrum. In this view, the X-ray-emitting accretion disk is partially obscured by multiple layers of gas. These absorbing layers would introduce complexity into the X-ray spectrum that could mimic the effects of rapid black-hole spin8. This has cast a cloud (literally) over measurements of the spins of supermassive black holes.

The new data from NuSTAR have finally resolved this issue, at least in this particular AGN, NGC 1365. Using the superior high-energy X-ray capabilities of NuSTAR, Risaliti et al. have produced a high-quality spectrum of the photons with energies in the range 3–80 keV. Above 10 keV, the signal-to-noise ratio of the spectrum is unprecedented and allows a direct face-off between the alternative, gas-absorption models and the more standard (spin-sensitive) models. The authors find that, for the gas-absorption models to work, the accretion disk would have to be blanketed by a thick layer of gas such that only 2–3% of the generated X-rays actually make it out of the system. Following this picture to its logical conclusion, the intrinsic emission in this AGN would have to be so luminous that the associated radiation pressure would blow the AGN apart. Although indirect arguments against gas-absorption models had been put forward previously5,9,10, this is by far the cleanest observational demonstration that such models fail.

With this cloud removed, it seems an unavoidable result that at least some supermassive black holes are spinning rapidly and must have grown in rapid accretion events. This raises fundamental theoretical questions about how gas is fed onto a supermassive black hole without fragmenting into smaller packets (or even stars) that would randomize its angular momentum3. These results also encourage us to push further and deeper with our X-ray observations, necessitating the development of more powerful X-ray observatories, so that we can use the diagnostic power of black-hole spin to uncover the story of supermassive-black-hole growth.

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Correspondence to Christopher S. Reynolds.

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Reynolds, C. Black holes in a spin. Nature 494, 432–433 (2013). https://doi.org/10.1038/494432a

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