Astronomers believe that black holes--those mysterious collapsed remnants of massive stars--are surrounded by invisible spheres called event horizons. Outside the event horizon, the pull of the black hole is just weak enough to let light or matter escape its clutches; but anything that crosses the horizon gets dragged swiftly into the hole. The region inside the event horizon is completely cut off from the rest of the universe, says Harvard astrophysicist Ramesh Narayan. That’s something unique to a black hole. Recently, Narayan and his colleagues found the most direct observational evidence of this process: they identified four star systems where radiation seems to be disappearing into the maw of a black hole.
Until now, proof of black holes’ existence had been inferred from the powerful gravitational pull on various stars by small, invisible objects. In such cases, only black holes or neutron stars--ultradense leftovers from burned- out supernovas--could seemingly account for the observed motions of the stars. Narayan and his colleagues Jeffrey McClintock and Michael Garcia decided to take a close look at a number of star systems that were likely hosts to black holes.
They used data from several satellites that are monitoring X-ray sources in the universe. In particular, they focused on star systems called X-ray novas. These systems are thought to contain a normal star being robbed of matter by an unseen but heavy companion. Every few decades this matter comes crashing down on the companion, releasing an intense burst of X-rays. The rest of the time, the siphoned-off matter just drizzles down, and little energy radiates away.
Astronomers theorize that the unseen companion can either be a black hole or a neutron star. Narayan says the way to tell the difference between the two would be to watch what happens not during intense bursts but during the quieter phases when the superhot star matter drizzles onto their surfaces. A neutron star would hold on to the matter, Narayan says, but radiate away the heat energy of the matter as X-rays. Like the neutron star, the black hole would pull in the matter from its neighbor. But because of the black hole’s intense gravity, energy would mostly be trapped inside.
Narayan and his colleagues carefully studied the X-rays emitted from nine relatively quiet X-ray novas. In most instances, they detected radiation levels up to a million times fainter than those produced during recent bursts. But in four cases, the level they found was barely detectable, amounting to less than a millionth of that released during comparable bursts. The only way to explain the X-ray shortfall, says Narayan, is if the energy is being swallowed up by a black hole. Says Narayan, We think we’re seeing the actual disappearance of energy through the event horizon.