NASA / CXC /M. Weiss (Harvard-Smithsonian Center for Astrophysics)

Spin zone.
Two artist's impressions showing (top) the dark and unknowable area inside the event horizon of a slowly spinning black hole, and the much smaller area near a rapidly spinning black hole.

Clocking Cosmic Eyewalls

Hurricane Katrina had nothing on GRS1915. This black hole seems to be rotating at least 950 times per second, hundreds of thousands of times faster than Katrina. The measurement marks the first time astronomers have been able to quantify the spin of a black hole so directly, and the findings could bring physicists closer to verifying the source of elusive cosmic phenomena known as gamma-ray bursts.

Much like hurricanes, black holes create a large swirling cloud, centered around a clear zone known as the event horizon. Unlike a hurricane's eye wall, however, nothing pulled inside an event horizon escapes, not even light. A black hole's enormous gravity can crush millions of stars worth of matter down to the size of a single electron. What's more, when a black hole really whirls, the ferocious gravitational energy created by that spin can propel gigantic jets of hot gas out of the cloud at nearly the speed of light and scorch the neighborhood with extremely powerful x-rays. So far, scientists have had to rely on theory and indirect measurements to determine how fast black holes can spin and, therefore, how much energy they can generate.

Now a research team at the Harvard Smithsonian Center for Astrophysics in Cambridge, Massachusetts, has used NASA's Rossi X-ray Timing Explorer satellite to measure the x-ray spectrum of gas jets emanating from GRS1915. The researchers focused on the central part of the black hole's accretion disk--a place called the innermost stable circular orbit, which is essentially the eyewall. Gas that ventures any closer to the event horizon is snatched in almost instantly and disappears. Combining the Rossi data with existing information about the black hole's mass allowed the researchers to compute the radius of this orbit--about 33 kilometers--and the hence the size of the event horizon--no bigger than 50 kilometers across. Those two figures yield the rate of the black hole's spin, they report in the 20 November issue of the Astrophysical Journal. "It's a relatively simple formula," says team leader Jeffrey McClintock, adding that the calculations show rapidly spinning black holes can pull accreting matter into a much tighter circle without sucking it into oblivion.

GRS1915 is, by black hole standards, fairly ordinary--larger than some, but far smaller than the monster lurking at the center of the Milky Way. That means "rapidly spinning holes probably are pretty common," says team member and theoretical astrophysicist Ramesh Narayan. He adds, however, that this conclusion is based only on a few observations, so the team needs to collect more spin rates to be sure.

Astrophysicist Chris Reynolds, of the University of Maryland, College Park, agrees. He says the new measurements are important, but he cautions that uncertainties remain in interpreting the data. Meanwhile, Stan Woosley, a theoretical astrophysicist at the University of California, Santa Cruz, says clocking black hole rotation accurately could help in the search for elusive cosmic phenomena known as gamma-ray bursts (ScienceNOW, 30 August). Woosley, who has constructed computer models of the bursts, says they probably are caused by rapidly spinning black holes, but up to now those spin rates have not been confirmed.

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