Tag Archives: AGN

Black holes in a spin

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.

Picture of NCC 1365

The Great Barred Spiral galaxy (NGC 1365) is about 56 million light years away in the constellation Fornax. (Credit: NASA)

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.

A new standard candle?

Standard candles have played a hugely important role in establishing the cosmological distance ladder. It’s easy to see why: the more distant something is the dimmer it appears, according to the inverse-square law. So if we know how bright something really is then, by measuring how bright it appears to be, we can determine its distance.

A standard candle appears dimmer the more distant it is

A standard candle appears dimmer the more distant it is
Credit: Karen Kwitter

Cepheid variables and Type Ia supernovae are perhaps the most well-known standard candles, and the study of these objects have transformed our understanding of the universe. But they (and the several other standard candles used in astronomy) are not without problems. One of the main difficulties is that we can’t see them over very large distances. Even Type Ia supernovae cannot be used to make reliable distance measurements beyond a redshift of about 1.7. So one of the most interesting astronomical results of 2011, at least in my opinion, was the surprising discovery of a standard candle that can work over truly cosmological distance scales: active galactic nuclei (AGNs).

Artist's impression of an accretion disc and torus around a black hole

An artist's impression of an accretion disc and torus around an AGN
Credit: NASA/CXC/M.Weiss

An AGN is one of the brightest objects in the universe and so can be seen over extreme distances. The power source for an AGN’s extreme luminosity is the supermassive black hole that lies at its centre. An accretion disc – a collection of matter that forms as matter spirals into a dense object – surrounds an AGN’s supermassive black hole. (See chapter 4 of New Eyes on the Universe for an explanation of accretion discs.) Further away from the black hole, at least with type-1 AGNs, lies a dense area of dust and gas known as the broad-line region. The region gets its name because the black hole’s gravitational influence whips the dust and gas around at high speed, and the Doppler effect causes emission lines to be broadened.

And how does this rather chaotic set-up generate a standard candle? Well, the broad-line region emits light because its gas has been ionised. The ionisation occurs because high-energy photons are emitted by the accretion disc and subsequently hit the region. The key point here is an accretion disc is a variable object: sometimes it ‘flares’. This makes it possible to compare the time at which the accretion disc emits light and the broad-line region re-emits light, and that time delay gives the radius of the broad-line region. What four astronomers – Darach Watson, Kelly Denney, Marianne Vestergaard and Tamara Davis – have found is that there’s a relationship between the size of the radius and the central luminosity of the AGN. They checked the relationship on a sample of 38 AGNs at a known distance and it seems that, although there is scatter in the data, the technique will work as a distance indicator. (You can read their paper at arxiv.)

The AGN standard candle is not as accurate as the Cepheid or supernova candles. But since AGNs can be seen over tremendous distances, and since they can be studied over long periods of time, it seems certain that the technique will become of increasing importance. In particular, a standard candle that lets astronomers measure distances directly up to a redshift of about 4 will provide a valuable tool for probing the nature of dark energy.