This week, the journal Science have produced a special issue dedicated to the subject of black hole research, and Rob Fender of the University of Southampton reviewed what we know about the X-ray emission of black holes.
Black holes are the collapsed end state in the lives of stars more massive than a few times the mass of the Sun. When these stars run out of fuel, no force can overcome the gravitational self-attraction of their cores, which consequently collapse down to an infinitely small point in an event called a Type II supernova explosion. The result is a black hole with a mass of a few times that of the Sun.
It is estimated that there are likely to be at least 100 million such black holes in our galaxy, based on the number of massive stars that we see today, and their estimated lifetime of a few million years. Yet although this makes them quite common objects, they are very difficult to observe as not even light can travel fast enough to escape from their surfaces.
The tell-tale sign of a black hole is X-ray emission, produced whenever gas falls in towards the event horizon. In the process of spiralling in towards its ultimate fate, this gas reaches incredibly high temperatures and densities as it is compressed into an ever smaller space. At temperatures of hundreds of millions of degrees, it doesn't just glow red hot, it glows X-ray hot. The energy required to heat gas to these temperatures is so great that almost all processes other than the accretion of material onto a black hole can be ruled out in such objects.
However, this X-ray emission is only produced if the black hole happens to have a source of gas falling onto it. Most of the Milky Way galaxy is a deep vacuum, with stars no closer than a few lightyears apart. So it is only those black holes that have close companion stars that can accrete gas at a significant rate, by gradually stripping the outer layers of gas from their companions.
Moreover, the accretion process is so violent that it is rarely sustained. Instead, black holes undergo fits of accretion, accompanied by explosive outbursts, which then prevent any further accretion from taking place for many months, or perhaps even many centuries. This means that in the past few decades of black hole research, we have only seen the tip of the iceburg of the Milky Way's black hole population.
Significant questions remain. We know that there is a supermassive black hole at the centre of the Milky Way which has a mass of several million times the mass of the Sun, and it appears that most other galaxies also have similarly sized black holes at their centres. How do these black holes acquire so much mass? They must have found an acretion mode which is much faster than simply munching on the outer envelopes of close companion stars.
Another interesting problem is that some galaxies, known as active galaxies, have intensely bright nuclei, understood to be powered by a flow of gas onto the central black hole. Others, like our own galaxy, appear not to have any flow of gas onto their central black holes. Why is this so? Do supermassive black holes have episodes of accretion similar to their stellar mass counterparts? It appears likely that they do, on a vastly enlarged scale.
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