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Extreme Black Hole Pushes Spin “limit”

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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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