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Giant black hole found to spin like mad

Feb. 28, 2013
Courtesy of the Harvard-Smithsonian Center for Astrophysics
and World Science staff

Im­ag­ine a ball more than two mil­lion miles wide—eight times the dis­tance from Earth to the Moon—spin­ning so fast that its sur­face is trav­el­ing at nearly the speed of light. 

Such an ob­ject ex­ists, sci­en­tists say: the black hole at the cen­ter of the spir­al gal­axy NGC 1365. As­tro­no­mers meas­ured its spin rate us­ing new da­ta from a space-based in­stru­ments.

In this artist's con­cep­tion a su­per­mas­sive black hole is sur­rounded by a hot "ac­cre­tion disk," while some in­spi­ral­ing ma­te­ri­al is fun­neled in­to a wispy blue je­t. New mea­sure­ments show that the black hole at the cen­ter of gal­axy NGC 1365 is spin­ning at close to the max­i­mum pos­si­ble rate. This sug­gests that it grew via "ordered ac­cre­tion" rath­er than by swal­low­ing ran­dom blobs of gas and stars. (Cred­it: NA­SA/JPL-Caltech)


“This is the first time an­y­one has ac­cu­rately meas­ured the spin of a su­per­mas­sive black hole,” the huge kind of black hole that sits at the cen­ters of ga­lax­ies, said as­t­ro­phys­icist Gui­do Ri­sal­iti, lead au­thor of a re­port on the re­sults.

A black hole is an ob­ject so heavy and com­pact that its gra­vity over­pow­ers and drags in an­y­thing near­by, even light. This gra­vity is so strong that, as the black hole spins, it drags the sur­round­ing space along. The edge of this spin­ning hole is called the event ho­ri­zon. 

Any ma­te­ri­al cross­ing the event ho­ri­zon is pulled in­to the black hole. Ma­te­ri­al spir­als in­wards and as it does so, it ac­cu­mu­lates in a disc-shaped ar­ea called an ac­cre­tion disk. There, fric­tion heats it and causes it to emit X-rays.

Risal­iti, of the Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics in Cam­bridge, Mass. and INAF-Ar­cetri Ob­serv­a­to­ry in Flor­ence, It­a­ly, and col­leagues meas­ured X-rays from the cen­ter of NGC 1365 to learn where the dis­c’s in­ner edge lies. Cal­cula­t­ions show that its loca­t­ion de­pends on the black hole’s spin. Since a spin­ning black hole dis­torts space, the disk ma­te­ri­al can get clos­er to the black hole be­fore be­ing sucked in.

The new mea­sure­ments, re­ported in the Feb. 28 is­sue of the jour­nal Na­ture, were made us­ing a space-based X-ray tel­e­scope called the Nu­clear Spec­tro­scop­ic Tel­e­scope Ar­ray, and the Eu­ro­pe­an Space Agen­cy’s XMM-Newton X-ray satel­lites.

As­tro­no­mers want to know the black hole’s spin for sev­er­al rea­sons. The first is phys­i­cal. Only two num­bers de­fine a black hole: mass and spin. By learn­ing those two num­bers, you learn eve­ry­thing there is to know about the black hole.

Most im­por­tant­ly, the black hole’s spin gives clues to its past and by ex­ten­sion the ev­o­lu­tion of its host gal­axy, as­tro­no­mers ex­plain. “The black hole’s spin is a mem­o­ry, a rec­ord, of the past his­to­ry of the gal­axy as a whole,” said Ri­sal­iti.

Al­though the black hole in NGC 1365 is cur­rently as mas­sive, or heavy, as sev­er­al mil­lion Suns, it was­n’t born that big. It's thought to have grown over bil­lions of years by ac­cu­mu­lating stars and gas, and by merg­ing with oth­er black holes.

Spin re­sults from a trans­fer of a prop­er­ty called an­gu­lar mo­men­tum, like play­ing on a chil­dren’s swing. If you kick at ran­dom times while you swing, you’ll nev­er get very high. But if you kick at the be­gin­ning of each down­swing, you go high­er and higher. You are adding an­gu­lar mo­men­tum.

Sim­i­lar­ly, if the black hole grew ran­domly by pulling in mat­ter from all di­rec­tions, its spin would be low, phys­i­cists say. Since its spin is close to the max­i­mum pos­si­ble, the black hole in NGC 1365 must have grown through “ordered ac­cre­tion” rath­er than mul­ti­ple ran­dom events, Risal­iti and col­leagues said.

Stud­y­ing a su­per­mas­sive black hole al­so al­lows the­o­rists to test Ein­stein’s the­o­ry of gen­er­al rel­a­ti­vity in ex­treme con­di­tions. Rel­a­ti­vity de­scribes how gra­vity af­fects the struc­ture of space-time, and no­where is space-time more dis­tort­ed than in the im­me­di­ate vicin­ity of a black hole.


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Imagine a ball more than 2 million miles across—eight times the distance from Earth to the Moon—spinning so fast that its surface is traveling at nearly the speed of light. Such an object exists, scientists say: the black hole at the center of the spiral galaxy NGC 1365. Astronomers measured its spin rate using new data from a space-based instruments. “This is the first time anyone has accurately measured the spin of a supermassive black hole,” the huge kind of black hole that sits at the centers of galaxies, said lead author Guido Risaliti. A black hole is an object so heavy and compact that its gravity overpowers and drags in anything nearby, even light. This gravity is so strong that, as the black hole spins, it drags the surrounding space along. The edge of this spinning hole is called the event horizon. Any material crossing the event horizon is pulled into the black hole. Material spirals inwards and as it does so, it accumulates in a disc-shaped area called an accretion disk. There, friction heats it and causes it to emit X-rays. Risaliti, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. and INAF-Arcetri Observatory in Florence, Italy, and colleagues measured X-rays from the center of NGC 1365 to learn where the disc’s inner edge lies. Calculations show that its location depends on the black hole’s spin. Since a spinning black hole distorts space, the disk material can get closer to the black hole before being sucked in. The new measurements, reported in the Feb. 28 issue of the journal Nature, were made using a space-based X-ray telescope called the Nuclear Spectroscopic Telescope Array, and the European Space Agency’s XMM-Newton X-ray satellites. Astronomers want to know the black hole’s spin for several reasons. The first is physical. Only two numbers define a black hole: mass and spin. By learning those two numbers, you learn everything there is to know about the black hole. Most importantly, the black hole’s spin gives clues to its past and by extension the evolution of its host galaxy, astronomers explain. “The black hole’s spin is a memory, a record, of the past history of the galaxy as a whole,” said Risaliti. Although the black hole in NGC 1365 is currently as massive, or heavy, as several million Suns, it wasn’t born that big. It grew over billions of years by accumulating stars and gas, and by merging with other black holes. Spin results from a transfer of a property called angular momentum, like playing on a children’s swing. If you kick at random times while you swing, you’ll never get very high. But if you kick at the beginning of each downswing, you go higher and higher. You are adding angular momentum. Similarly, if the black hole grew randomly by pulling in matter from all directions, its spin would be low, physicists say. Since its spin is so close to the maximum possible, the black hole in NGC 1365 must have grown through “ordered accretion” rather than multiple random events, Risaliti and colleagues said. Studying a supermassive black hole also allows theorists to test Einstein’s theory of general relativity in extreme conditions. Relativity describes how gravity affects the structure of space-time, and nowhere is space-time more distorted than in the immediate vicinity of a black hole. The team also has additional observations of NGC 1365 that they will study to determine how conditions other than black hole spin change over time. Those data are currently being analyzed. At the same time, other teams are observing several other supermassive black holes.