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Extreme black hole pushes spin “limit”
Nov. 21, 2006
Courtesy Harvard-Smithsonian Center for Astrophysics
and World Science staff
Physicists have measured a black hole spinning so quickly—more than 950 rotations per second—that it pushes
the theoretical speed limit for this process, a study reports. A spinning black hole. Click image to view animation (6 Mb. Credit:
NASA / Honeywell Max-Q Digital Group / Dana Berry)
Black holes are a prediction of Einstein’s Theory of General Relativity. When any mass, such as a star, becomes sufficiently compact, its own
gravity crushes it into a point, and becomes so potent that even light can’t escape its grip.
This is called a black hole.
Black holes are the sites of strange happenings, and that’s even truer of rapidly spinning
ones, said astronomer Jeffrey McClintock of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
“This regime of gravity is as far from direct experience and knowing as the subatomic world itself,” he said.
Using a spin-measurement method developed by McClintock and the center’s
Ramesh Narayan, the team used data from a NASA satellite called the Rossi X-ray Timing Explorer to get what they called the most direct
determination to date of black hole spin. The findings appear in the Nov. 20 issue of the
Astrophysical Journal.
“We now have accurate values for the spin rates of three black holes,” said
McClintock. “The most exciting,” he added, is the result for a black hole designated GRS1915+105. Its measured spin is between 82 percent and 100 percent of the theoretical maximum.
This “has major implications for explaining how black holes emit jets, for modeling possible sources of gamma-ray bursts, and for the detection of gravitational waves,” said
Narayan. Gravitational waves are ripples in space-time predicted by Einstein, and believed to come from exotic processes such as merging black holes and collapsing stars. Gamma-ray bursts are
blasts of high-energy radiation that can be momentarily the brightest flashes in the universe.
Theoretical astrophysicist Stan Woosley of the University of California, Santa Cruz, has theorized that these bursts also result from the collapse of massive stars.
His models, however, depend on the existence of very high-spin black holes, until now never confirmed.
For that reason, the new study “is extremely important,” Woosley said. “I had no idea such measurements could be made.”
The paper concludes that GRS 1915 and two other black holes studied
were born with their high spins. In other words, the rotational momentum of the original massive star became that of the black hole.
Astronomers care about black hole spin because it’s one of just two fundamental quantities that describe the objects completely, said
McClintock. The other is mass. “We know of nothing else this simple except for a fundamental particle like an electron or a quark,” he added. But whereas astronomers have measured black hole mass, he said, it’s been much harder to measure spin.
In fact, “until this year, there was no credible estimate of spin for any black hole,” said
Narayan.
A black hole’s gravity is in theory so strong that, as it spins, it drags the surrounding space along. The edge of this spinning hole is called the event horizon. Any material crossing the event horizon sinks inexorably into the black hole.
The rotation the team measured “is the rate at which space-time is spinning, or is being dragged, right at the black hole’s event horizon,” said
Narayan.
The high-speed black hole is the most massive of 20 black holes of a type
called X-ray binaries with known masses, the researchers said. They’re
thought to weigh about as much as 14 Suns
An X-ray binary is a system in which two objects orbit each other, and gas from one—a normal star like the Sun—gets sucked gravitationally into the other, in this case, a black hole. In recent decades, dozens of black holes have been discovered in X-ray binary systems, scientists say.
In these dances, the gas heats up to millions of degrees and radiates X-rays
as it spirals onto the black hole. Characteristics of these rays can
be used to gauge the black hole spin, according to McClintock and colleagues. That’s what was done for this object, they added, which is also noted for bizarre properties such as rapidly fluctuating X-ray emissions and nearly light-speed ejections of jets of matter.
The measurement technique is based on Relativity, the researchers
explained. Gas that accumulates onto a black hole radiates only until it reaches the event horizon. Past that, the radiation itself can no longer escape the
black hole. The distance from the black hole center to the event horizon depends on spin rate.
The distance in turn affects the brightness and temperature of the emissions, because the shorter the distance, the hotter they are. Thus these
properties of the X-rays, the physicists said, give an estimate of the spin rate.
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Physicists have measured a black hole spinning so quickly—more than 950 rotations per second—that it pushes the predicted speed limit for this process, a study reports. Black holes are a prediction of Einstein’s Theory of General Relativity. When any mass, such as a star, becomes sufficiently compact, its own gravity crushes it into a point. This is called a black hole, an object of such crushing gravity that even light can’t escape its grip. Black holes are the sites of strange happenings, and that’s even truer of rapidly spinning black holes, said astronomer Jeffrey McClintock of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. “This regime of gravity is as far from direct experience and knowing as the subatomic world itself,” he said. Applying a technique to measure spin developed by McClintock and the center’s Ramesh Narayan, the team used data from a NASA satellite called the Rossi X ray Timing Explorer to get what they called the most direct measurement to date of black hole spin. The results appear in the Nov. 20 issue of the Astrophysical Journal. “We now have accurate values for the spin rates of three black holes,” said McClintock. “The most exciting,” he added, is the result for a black hole designated GRS1915+105. Its measured spin is between 82% and 100% of the theoretical maximum. This “has major implications for explaining how black holes emit jets, for modeling possible sources of gamma-ray bursts, and for the detection of gravitational waves,” said Narayan. Gravitational waves are ripples in space-time predicted by Einstein, and believed to come from exotic processes such as merging black holes and collapsing stars. Gamma-ray bursts are short-lived bursts of high-energy radiation that can be momentarily the brightest flashes in the universe. Theoretical astrophysicist Stan Woosley of the University of California, Santa Cruz, has theorized that these bursts also result from the collapse of massive stars. These models, however, depend on the existence of very high-spin black holes, until now never been confirmed. For that reason, the new study “is extremely important,” Woosley said. “I had no idea such measurements could be made.” The paper concludes that GRS 1915 and two other black holes studied by the team were born with their high spins. In other words, the rotational momentum of the original massive star became that of the black hole. Astronomers care about black hole spin because it’s one of just two fundamental quantities that describe the object completely, said McClintock. The other one is its mass. “We know of nothing else this simple except for a fundamental particle like an electron or a quark,” he added. But whereas astronomers have measured black hole mass, he said, it’s been much harder to measure spin. In fact, “until this year, there was no credible estimate of spin for any black hole,” said Narayan. A black hole’s gravity is in theory so strong that, as it spins, it drags the surrounding space along. The edge of this spinning hole is called the event horizon. Any material crossing the event horizon sinks inexorably into the black hole. “The black hole spin frequency we measured is the rate at which space-time is spinning, or is being dragged, right at the black hole’s event horizon,” said Narayan. The high-speed black hole is the most massive of 20 black holes of a type designated as X-ray binaries, of known masses, weighing about as much as 14 Suns, the researchers said. An X-ray binary is a system in which two objects orbit each other, and gas from one—a normal star like the Sun—gets sucked gravitationally into the other, in this case, a black hole. In recent decades, dozens of black holes have been discovered in X-ray binary systems, scientists say. In X-ray binaries, the gas spirals onto the hole, heating up to millions of degrees and radiating X-rays in the process. Characteristics of these releases can be used to determine the black hole spin, according to McClintock and colleagues. That’s what was done for this object, they added, which is also noted for bizarre properties such as rapidly fluctuating X-ray emissions and nearly light-speed ejections of jets of matter. The technique is based on a prediction of Relativity, the group explained. Gas that accumulates onto a black hole radiates only until it reaches a certain distance from the hole, called the event horizon. Past that, the radiation itself can no longer escape the hole. The spin rate directly affects this critical distance. That, in turn, influences the brightness and temperature of the emissions, because the shorter the distance, the hotter they are. Thus these properties of the X-rays give an estimate of the spin rate.