Tag Archives: black hole

Alien megastructures

For the past five decades the search for extraterrestrial intelligence has been dominated by the search for radio signals. There are good reasons why this search strategy makes sense, but the available search space is so vast (our dishes have to be pointing in the right direction at the right time, and tuned to the right frequency) that the phrase “radio SETI” is an excellent synonym for the phrase “looking for a needle in a haystack”. Are there any other options for the search?

Personally, I believe that we need to adopt a Stapledonian approach to the problem.

Olaf Stapledon, a British philosopher and science fiction author, considered what might happen to intelligence in the distant future. For example in one of his novels, Star Maker, published in 1937, he described what we now call Dyson spheres: structures that orbit a star and enable a civilisation to utilise most of the energy output of its parent star. The creation of a Dyson sphere is far beyond our present technical capabilities, but who knows what we (or our mind-children) will be able to achieve a thousand years from now, or ten thousand years from now, or a hundred thousand years from now. And 100,000 years is an eye blink in cosmic terms; if there are extraterrestrial intelligences out there then they might be millions of years in advance of us. Thus in a Stapledonian approach to SETI we would look for examples of astroengineering, or megastructures that could only have been developed by technologically advanced intelligences. The detection of such a megastructure wouldn’t open up the possibility of communication, as a traditional radio SETI detection might do, but it would at least tell us that we were not alone. That in itself would be a terrifically important discovery.

The recent furore surrounding KIC 8462852 is an example of how a Stapledonian approach is starting to appear in the ongoing search for extraterrestrial intelligence. KIC 8462852 is an F-type main-sequence star about 1480 light years away from Earth in the constellation Cygnus. The Kepler space telescope recorded fluctuations in the light from the star – but a recent paper demonstrated that the fluctuations were so bizarre that they could not come from a transiting exoplanet. For one thing, the dimming of the starlight is not periodic; for another, the dimming is extreme (15% in one episode, 22% in another; for comparison, a Jupiter-sized planet blocks about 1% of its star’s light). So what is causing this weird behaviour? We don’t know. The authors of the paper suggest that the dimming might be caused by a series of comets, surrounded by clouds of gas, perturbed from their usual orbits by the gravitational influence of a nearby star; a small red dwarf close to KIC 8462852 might be the culprit. It’s possible. But it’s far from certain that this story can explain all the features that are seen. Any other explanations? Well … could it be that we have caught an advanced alien civilisation in the act of building a Dyson sphere? I doubt it. I REALLY doubt it. Just because we observe something we can’t immediately explain we shouldn’t immediately attribute it to alien intelligence (remember that, for a short while, the radio signals from pulsars were thought to be evidence for ETI; astronomers soon figured out the true explanation for the radio emissions). Nevertheless, it surely can’t harm to follow up these observations of KIC 8462852 with traditional radio-based SETI observations.

Several astronomers have already searched for the infrared emission that would accompany a Dyson sphere. But the search for Dyson spheres would form only a small part of a Stapledonian approach to SETI. We need to use our imagination and try to envisage the sorts of technology that a truly advanced civilisation might develop. In a previous post I looked at how John Smart’s transcension hypothesis argues that black holes are an attractor for intelligence. The philosopher Clément Vidal adopts a related approach. (Incidentally, in my book I wrote that is Belgian. He works in Belgium, but is in fact French. Apologies, Clément!)

Clément uses a two-dimensional metric, first proposed by John Barrow, to describe advanced civilisations. The Kardashev metric is well known: K1 civilisations control the energy output of their home planet, K2 civilisations control the energy output of their home star, K3 civilisations control the energy output of their home galaxy. But Barrow pointed out that there is a scale of inward manipulation that might be just as applicable to extraterrestrial civilisations: a B1 civilisation can manipulate the universe at the 1m level; a B2 civilisation can work at the 10–7m scale; a B3 civilisation manipulates at the nanoscale; and a BΩ civilisation can manipulate spacetime at the Planck level. In a 2011 paper Clément talks about the possibility of K2-BΩ civilisations; he has since switched to a more memorable appellation “stellivore”.

If we accept that stellivores could exist, an obvious question is: what might stellivores be doing that our telescopes and instruments might pick up?

Well, a stellivore might possess a technology involving black holes. (I won’t go here into the many reasons they might want to use black holes. Suffice it to say that a Stapledonian mindset would consider black holes to be a natural endpoint for many technologies.) And we know that in principle it is possible to detect activity around black holes; we know this because astronomers have already investigated X-ray binaries (XRBs). In an XRB a donor object (typically a star) loses material to a compact accretor (typically a black hole). The infalling matter releases huge amounts of gravitational potential energy. So could XRBs provide evidence for stellivores? In my book I write that we could “look for evidence for the regulation of energy flow within XRBs”. As Clément points out, there’s already evidence for such regulation; the key question – just as it is with KIC 8462852 – is whether the observations are best interpreted in terms of astrophysics or astrobiology?

Since Clément’s 2011 paper he has extended his vision to include a wider range of XRBs: the stellivore family could include cataclysmic variable X-ray pulsars, for example, with black holes being the end stage.

To my mind, the great thing about this Stapledonian sort of approach is that it broadens the range of techniques we can apply when searching for signs of extraterrestrial intelligence. Traditional radio-based SETI has its place. But the ideas of John Smart and Clément Vidal tell us that we could also profitably search at the highest energies.

Measuring the distance to black holes

It’s not often that you come across a new method of distance determination in astronomy, but today’s Nature contains a paper (“A dust-parallax distance of 19 megaparsecs to the supermassive black hole in NGC 4151” by Sebastian Hönig, Darach Watson, Makoto Kishimoto and Jens Hjorth) that describes a method for directly determining the distances to quasars and galaxies with active nuclei.

As I’m sure you’re aware, over the course of many decades astronomers have developed a “cosmological distance ladder”. If astronomers understand an object – how bright it really is, how big it really is – they can determine its distance by measuring how bright or how big it appears. However, each time astronomers step up one rung on the distance ladder they introduce a source of error. It’s inevitable. A much better way of measuring distance is to use a direct method: to use geometry, in other words.

The most familiar example in astronomy of distance determination through geometry is that of annual parallax. As Earth moves around the Sun, the position of a nearby star is seen to shift relative to the background of the more distant, “fixed” stars. Draw lines between Earth, Sun and star and we generate a huge triangle. But we can measure the angular shift, and we know the diameter of Earth’s orbit, so we have all the information we need to solve the triangle. (Assuming we’ve done basic geometry in school.)

Annual parallax

Earth, Sun and star form a triangle. We know the base of the triangle and we can measure the parallactic shift caused by Earth’s motion. We can solve the triangle and determine the star’s distance.

If a star moves by 1 second of arc then by definition it would be at a distance of one parsec. All stars (except the Sun, of course) are further away than 1pc and so all angular shifts exhibited by stars are less than 1 second of arc. Indeed, although this method formed one of the earliest and most important rungs in the cosmological distance ladder, it is difficult to apply it to very distant objects because the angles involved are simply too small to measure.

How, then, can Hönig and his colleagues apply a geometrical technique to a galaxy that lies 19 million parsecs away? Aren’t the angles way too small to measure?

Well, these astronomers have “inverted” the familiar parallax triangle. The base of the triangle isn’t the diameter of Earth’s orbit, it’s size of a region surrounding an active galactic nucleus (in this particular case, it’s the size of a dust region surrounding the supermassive black hole in the nucleus of the galaxy NGC 4151).

NGC4151

The Seyfert galaxy NGC 4151 lies 62 million light years from Earth. In this image, blue is from X-ray observations; yellow dots are from optical observations; and red is from radio observations. (Credit: NASA, ESA)

In order to solve the triangle formed by Earth, the supermassive black hole in NGC 4151, and the dust region surrounding the black hole, astronomers need to measure two things: (i) the base of the triangle – in other words, the true distance between the black hole and the dust, and (ii) the smallest angle in the triangle – in other words, angular size of the dust cloud.

The distance between the black hole and the dust cloud is easy to measure in principle – though difficult and messy in practice. As matter falls towards the black hole it heats up, so infalling matter produces radiation from a region just outside the event horizon. (Note that, although the black hole has a huge mass the radius of the event horizon is small. This is not a big object.) The radiation spits, and flickers, and flares: it’s highly variable. So suppose there’s a flash of light from just outside the event horizon. Some of the light will take a path directly towards our telescopes; some of the light will head of at right angles and continue until it interacts with dust clouds. This interaction will cause the dust to light up (or “reverberate”), which our telescopes will detect some time after the detection of the initial flash. By measuring the time delay astronomers can thus calculate the length of the base of the triangle (it’s just the delay multiplied by the speed of light). The technique is called “reverberation mapping”.

The angular size of the hot dust clouds that surround active galactic nuclei can be determined with a sufficiently precise interferometer. The Keck interferometer has sufficient resolution to measure the angular size the dust clouds in NGC 4151, and this is what Hönig and his colleagues did. By using the angular size determined by the Keck interferometer with lengths determined by a previous reverberation mapping project they were able to solve the NGC 4151 triangle: it’s 19 Mpc away (give or take 2.5 Mpc).

This distance comes from geometry. There’s no chain of inference involved as there is with the cosmological distance ladder: geometry gets you there directly.

The method has great potential because active galactic nuclei are bright enough to shine all the way across the universe. If we were to develop interferometers with increased resolving power (rather than just developing telescopes with ever-greater light-collecting ability) then we would have the ability to use geometry to measure distances over cosmological scales.

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.

Weighing a black hole

At the heart of every galaxy there seems to be a supermassive black hole; that much astronomers are pretty sure about. What’s much less well understood is the possible connection between the mass of the central black hole and the size of the galaxy’s bulge; there are hints that supermassive black holes and galaxies might co-evolve, but the details are unknown. One of the barriers towards understanding this possible relationship is the difficulty in measuring black hole masses. It’s not impossible to measure the mass of a supermassive black hole: you can track stars as they orbit the black hole, for example, and use their motion to deduce the mass. Or, even better, you can track the motion of ionised gas clouds. In a few rare cases, as I mentioned in Measuring the Universe, it’s possible to observe maser emission from the central regions and from this measure a mass. These methods only work for nearby galaxies, however.

Timothy Davis, Martin Bureau, Michele Cappellari, Marc Sarzi and Leo Blitz, in a paper published in the 30~January~2013 issue of Nature (A black-hole mass measurement from molecular gas kinematics in NGC4526) have developed a new technique for measuring the mass of a supermassive black hole and used it to estimate the mass of the central black hole in NGC4526.

The team’s idea is to measure the motion of molecular gas clouds by observing emission from carbon monoxide (CO). They tested their idea by observing the CO(2-1) emission line from gas in the lenticular galaxy NGC 4526, which is about 17 Mpc away from us in the Virgo cluster. The wavelength from such CO emission is 1.3 mm, which means that millimetre-wavelength telescopes are required. The team used the 23-telescope Combined Array for Research in Millimetre Astronomy (CARMA) to observe the emission, and from their observations deduced a central black hole mass of about 450 million solar masses.

NGC4526

The lenticular galaxy NGC4526 (Credit: NASA)

What’s interesting about this paper is not the result itself, but the possibilities it opens. The team needed 100 hours of observing time with CARMA to get their result. With ALMA, however, the same job would take less than an hour: observations become possible that were previously impractical. When it becomes fully operational ALMA, using this technique, may well provide us with an understanding of how galaxies and their supermassive black holes live together.

Arrivederci Rossi

On 3 January 2012, one of the most productive X-ray observatories of all time ceased its science operations. The Rossi X-ray Timing Explorer (RXTE) observed the high-energy cosmos for 16 years. During that time RXTE made a host of discoveries, and its results were used in about 2000 refereed publications. My favourite Rossi finding? The discovery of a black hole with a mass just 3.8 times that of the Sun!

Rossi, fully assembled, waiting for launch at the Kennedy Space Center

Rossi, fully assembled, waiting for launch at the Kennedy Space Center
Credit: NASA

Artist's impression of the Rossi X-ray Timing Explorer

Artist's impression of the Rossi X-ray Timing Explorer
Credit: NASA

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.