Toying with space-time
Black holes, which are only theoretical but are believed to exist by nearly all astronomers, come in many sizes. Supermassive black holes are thought to reside at the centers of most or all galaxies.
Less massive objects, called stellar black holes, seem to be distributed throughout galaxies like our own Milky Way. They are formed when massive stars exhaust their fuel supplies and explode their outer shell of gas in an event called a supernova. The remaining bulk, still several times more massive than the Sun, collapses into a black hole.
The new study involves a stellar black hole called GRO J1655-40, roughly 10,000 light-years from Earth and estimated by other research to be roughly seven times as massive as our Sun.
This mass implies an innermost stable orbit of 64 kilometers (40 miles) if the black hole were not spinning, according to the study. But a spin rate of 450 times per second implies that incoming blobs could cozy up to within 49 km (about 30 miles).
The only way that gas can maintain a stable orbit that close to the black hole, Strohmayer said, is if the black hole is spinning.
"A spinning black hole modifies the fabric of space-time near it," Strohmayer said. "The spinning allows matter to orbit at a closer distance than if it were not spinning, and the closer matter can get, the faster it can orbit."
Matter approaching the event horizon of a spinning black hole can reach roughly half the speed of light, he said, or 20 to 30 percent faster than matter spinning into a non-rotating black hole.
Lynn R. Cominsky, a professor of physics and astronomy at Sonoma State University in California, said the study's data was solid that and that the black hole in question has the best estimation of mass for any known black hole.
"We have suspected that black holes spin but it's very hard to get direct evidence of anything that black holes do," said Cominsky, who was not involved in the study.
The spin on spin
How does a black hole get its spin? From the star out of which it formed, said Strohmayer. A basic law of physics, in fact, requires it. Because a star spins, it has a certain angular momentum.
"As the core of that star collapses to form the black hole, its angular moment is conserved," Strohmayer explains. "That black hole, while you can't think of it as having a surface that's spinning around, has associated with it some angular momentum."
GRO J1655-40 is more accurately known as a microquasar, a specific type of black hole with jets of high-speed particles shooting perpendicularly from the plane of matter that orbits it. It is these jets, and a black hole's gravitational effects on surrounding stars and matter, that allows researchers to identify black holes.
Similar findings in neutron stars
The pulse of the X-rays related to an incoming blob of matter was accompanied by a second, less rapid measurement thought to be related to the spinning of the black hole itself. That marks the first time such "twin peaks" have been found for black holes, Cominsky said.
Similar dual emissions had previously been identified with neutron stars -- incredibly dense and sometimes rapidly rotating stellar corpses that were not quite dense enough to become black holes.
But unlike a black hole, neutron stars do have a solid surface, and it has been thought that the X-ray emissions resulted from an interaction at the surface. Strohmayer said the new finding may therefore force changes in theories about neutron stars -- it's possible that the emissions come from somewhere above the surface.
The study used data collected by NASA's Rossi X-ray Timing Explorer.
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