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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.

Click to view animation

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.