Black Holes Really Are Black!
Black holes get their name because - at least according to the theory - they are so dense that no light can escape from them. However, there has so far been no direct demonstration of the fundamental property that they have no surface, and are thus genuinely black. Now research at Leicester University done by Professor Andrew King, with co-workers Dr Ulrich Kolb and Dr Ewa Szuszkiewicz, has shown for the first time that black holes really are black! Professor King will talk about this conclusion on Wednesday 9th April at the UK's National Astronomy Meeting at the University of Southampton.
There is strong indirect evidence that several binary star systems in our Galaxy contain black holes closely orbiting a normal companion star. The binary systems in question are unusual in that they emit huge amounts of X-rays in sporadic outbursts. These outbursts last for months, but may take as long as 50 years or more to recur. The outbursts are powered by the gravitational energy released when huge masses of gas from the companion star fall on to the star astronomers believe is a black hole. The gas is being steadily torn off the companion star by the black hole's gravitational pull. However it does not fall directly on to the black hole, but orbits it in the form of a large flat disc of gas, rather like Saturn's rings. The outbursts occur when this disc becomes unstable, and its gas suddenly falls towards the black hole.
Gas can build up in the disc and cause periodic outbursts only if the disc is normally fairly cool. Professor King's team has shown that, if there is a hot star at the centre of the disc, it will keep the disc hot. Under these circumstances, the instability never gets a chance to build up because material can flow steadily in towards the central star. The system then appears as a steady source of X- rays instead of having outbursts. There are many systems like this known: the central star in them is not a black hole but a neutron star. Neutron stars are almost as dense as black holes, but, crucially, they are just extended enough to have a real stellar surface. This surface is heated by the steady infall of gas from the disc, keeping the disc hot and stable, which in turn maintains the steady inflow heating the stellar surface in the first place.
Yet in every case where astronomers suspect for independent reasons that the central star is not a neutron star but a black hole, the system has outbursts rather than being steady. This must mean that the central star cannot heat the disc, i.e. that this star has no hot surface.
The inevitable conclusion is that black holes really are black. And this reasoning dispels any remaining doubt about whether the suspect black holes are the real thing.
This work was performed in the Department of Physics and Astronomy at the University of Leicester. The group is led by Professor Andrew King, with co-workers Dr Ulrich Kolb and Dr Ewa Szuszkiewicz.