Using incredibly dense but detectable neutron stars as a guide, researchers announced today they have found more evidence to bolster the case that black holes exist. The work also illuminates why black holes are so dark.
Black holes and neutron stars are like cousins, related in origin but with vastly different personalities. Both are incredibly dense and often live near a large companion star from which they siphon gas and at which they tug relentlessly with immense gravity. And both, at least at times, emit tremendous amounts of X-ray energy -- a signature that helps researchers find them.
But while neutron stars wear their characteristics on their sleeves, black holes theoretically hide everything in a dark warp of space-time.
Now a team of researchers has come up with a novel approach for comparing neutron stars to black holes. It has yielded more evidence of an "event horizon," a theorized sphere around a black hole that is the point of no return for matter and light. Significantly, the comparison helps to prove the existence of black holes, which to date are only theoretical objects.
Albert Einstein's theory of relativity predicted event horizons, and proving they exist would effectively prove that black holes are real.
The scientists used the Chandra X-ray Observatory to study energy output from a dozen dormant X-ray novae, a special class of objects that occasionally erupt as brilliant X-ray sources and then settle into decades of relative inactivity. The team discovered that the sources with presumed black holes as companions emitted only 1 percent as much energy while dormant as did the X-ray novae with neutron stars as companions.
"The most straightforward explanation of these observations is that the black hole candidates we have studied have event horizons that swallow just about all of the energy that surrounds them," said Stephen Murray of the Harvard-Smithsonian Center for Astrophysics. "Indeed, one could even say that this work shows why black holes deserve to be called black."
Disappearing act
Neutron stars and black holes are both thought to be collapsed stars. If the relic is a neutron star, the in-falling matter would release energy upon smacking into the surface. A black hole, on the other hand, has no surface -- just a point of infinite density surrounded by the event horizon -- so there is no matter for the surface to strike.
"Watching matter flowing into a black hole is like sitting upstream of a waterfall and watching the water seemingly vanish over the edge," said Ramesh Narayan, chairman of the Harvard Astronomy Department. "However, if the waterfall were replaced by a dam -- the analog of a neutron star surface -- then the water would pile up and one would see a mighty lake."
The scientists created two artist's renderings to illustrate what they think is happening. (See the click-to-enlarge images near the top-right of this page.)
In the case of the suspected black hole, gas from a companion star is drawn by gravity onto the black hole in a swirling pattern. As the gas nears the event horizon, a strong gravitational redshift makes it appear redder and dimmer. When the gas finally crosses the event horizon, it disappears, making the inner region appear black.
With the neutron star, gas from a companion star also flows inward. A similar gravitational redshift makes the gas appear redder and dimmer. But when the gas strikes the solid surface of the neutron star, it glows brightly.
"The comparison of black holes and their close cousins, the neutron stars, may be the most promising way to get a handle on the event horizon," said Jeffrey McClintock, also of the Center for Astrophysics.
The findings were presented at a meeting of the American Astronomical Society meeting in San Diego.
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