Black holes really are holes, astronomers say

Black holes really are holes – objects without a surface – say Drs Christine Done and Marek Gierlinski in a paper accepted for publication in the Monthly Notices of the Royal Astronomical Society. Having an “event horizon” rather than a surface is the property that makes something a black hole but, by definition, it’s impossible ever to see one directly. However, these new results give direct evidence of the existence of such holes in spacetime.

Drs Done and Gierlinski set out to track down the signatures of event horizons by looking for differences between objects thought to be black holes and collapsed bodies of a different kind that are only slightly less extreme – neutron stars. Any material captured by the strong gravity of either type of object will spiral inwards in much the same way, reaching speeds of up to half the speed of light and transforming some of the immense gravitational energy into X-ray emission. The crucial difference is that, in the case of a black hole, material should simply pour into the hole, taking its remaining energy with it and disappearing forever, whereas with a neutron star material smashes onto the surface, releasing whatever energy is left. As a consequence, the X-ray emissions from neutron stars and black holes should look different.

“The idea is simple in theory, and has been known for a long time, but until now it has been hard to put into practice because the X-ray emission even from a single type of object can show a bewildering variety of properties that are not well understood,” says Chris Done.

Only recently, with over 6 years of operation of NASA’s Rossi X-ray Timing Explorer, an orbiting X-ray observatory, have there been enough data to cover the whole range of behaviour of both neutron stars and black holes. Drs Done and Gierlinski took advantage of this enormous dataset, reducing all the information contained in the complex X-ray spectra down to just two numbers, which describe the slope of a spectrum at low and high energies. Plotting these two numbers against each other as an X-ray source changes in brightness gives a good overview of how the shape of the object’s X-ray spectrum is changing. “What we see is that neutron stars and black holes behave in distinctly different ways as more material falls onto them,” says Chris Done, “and the only big difference we know of that can account for the observations is that neutron stars have a surface, while black holes don’t.”