You may not realize it, but the sky is littered with black holes – regions of space where gravity is so strong that nothing, not even light, can escape. In the Milky Way, for instance, black holes a few times more massive than the Sun grow by accreting gas from companion stars within binary systems. At the centre of most galaxies, however, lurk black holes that are millions or billions of times more massive than the Sun, accreting gas that has been lost by stars or that has fallen into the galaxy.
It is this type of black hole that is of particular interest to scientists. The enormous pull of these "galactic nuclei" makes them ideal for studying the effects of strong gravity as predicted by general relativity. They can also tell us about the formation of galaxies. For instance, astrophysicists think that the energy released as matter falls into a black hole could be enough to blow all of a galaxy's dusty gas into deep space, depriving the galaxy of its star-forming material and thereby limiting its mass.
But black holes at the centre of galaxies are not easy to study. Many are inactive, which means that the luminosity of the radiation is simply too low to be detected and the objects remain hidden from view. However, about 10% of galactic nuclei are active, although even these are hard to spot. The very gas that falls into the black hole and releases gravitational energy also blocks much of the radiation subsequently given off. It is only at "hard" X-ray wavelengths – where photons have energies of tens to hundreds of kilo-electronvolts (keV) – that the radiation can easily penetrate the gas and reveal the presence of the active galactic nuclei (AGNs).
These hard X-rays can only be seen with advanced detectors and so observational data about AGNs have until recently been patchy. But NASA's Swift satellite, launched in 2004 in partnership with astronomers from Italy and the UK, has recently given us a comprehensive survey of over 150 AGNs within 400m light-years.
Andy Fabian tells the full story in the December issue of Physics World.