It takes a very good telescope to "see" a pit of absolute blackness. Yet in January researchers announced they had "seen" the boundary of a black hole, the infamous event horizon from which nothing ever returns.
"It is a bit odd to say we've discovered something by seeing almost nothing at all--less than the smile of the Cheshire cat, so to speak," acknowledged Michael Garcia of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass., "but, in essence, this is what we have done."
Essential to the discovery were the novel semiconductor X-ray sensors installed in a telescope aboard NASA's Orbiting Chandra X-Ray Observatory. Garcia and his colleagues used these charge-coupled devices (CCDs) to study the X-ray emissions of celestial objects [see images below], the most intriguing of which may be the X-ray novae. Each of these objects comprises a sun-like star and a collapsed star, which could be a black hole or a neutron star. If the two stars are close enough, the gravity of the collapsed star pulls a stream of gas from its companion "sun," and a brilliant burst of X-rays erupts as the gas is heated to very high temperatures. Often, periods of intense X-ray activity are separated by long periods of dormancy, during which much less radiation is emitted.
Garcia's team found that during dormancy the energy emitted by X-ray novae with black holes is markedly different from that emitted by novae containing neutron stars: the black hole type emits only 1 percent as much energy as the neutron star type. The most straightforward explanation, according to the scientists, is that the black holes are surrounded by event horizons that suck in all the energy incident upon them. Below the event horizon, the gravitational force is so great that nothing--not even light--can escape. "One could even say that this work shows why black holes deserve to be called 'black,'" said team member Stephen Murray. The finding, presented at a meeting of the American Astronomical Society in January of this year, confirms the deduction derived from Einstein's theory of general relativity by German astronomer Karl Schwarzschild back in 1916.