Scientists have clocked a black hole whipping space itself around and around at nearly the speed of light.

Image/animation right: A spinning black hole. Click image to view animation (6 Mb -- no audio). Credit: NASA/Honeywell Max-Q Digital Group/Dana Berry

Envision a merry-go-round, or a moving walkway. It is the fabric of space itself that is whizzing along. Step on this merry-go-round, and you too would be transported at near light speed.

A black hole's gravity is so strong that, as the black hole spins, it can pull space along for the ride. Yet measuring a black hole's spin rate has long eluded scientists. Twisted space is difficult to detect and perhaps even harder to fathom, like a modern art painting of black swirls on a black canvas.

Now a group led by Jeff McClintock and Ramesh Narayan of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., has a new technique to measure black hole spin, which they describe as surprisingly straightforward and direct. They have accurately measured one black hole's spin and are moving on to others.

In doing so, they hope to accurately test some of Albert Einstein's wildest predictions.

A Hole In Space

A black hole is an object so dense and with gravity so strong that nothing, not even light, can escape its pull if it ventures too close. Our Milky Way galaxy is dotted with millions of black holes, the majority of which cannot be detected. These black holes are the remains of massive stars, long since extinguished.

When a star at least 10 times more massive than our sun runs out of nuclear fuel to burn, it no longer has the energy to support its own mass. The core implodes in a fraction of a second, creating shock waves that cause the outer shells of the star to explode into space, an event called a supernova.

If there's enough mass in the core, nothing can stop its collapse. It just gets denser and denser, tinier and tinier. All the mass folds into a single point of infinite density. There is no longer any star surface left, just a spinning hole in space. The spin from the old star gets converted directly into the spin of the black hole.

The spherical border around this spinning hole is called the event horizon. The radius (distance from the event horizon to the black hole center) is only about 10 to 50 kilometers, or about 5 to 30 miles. Once anything crosses the event horizon, it falls down the slippery slope into the black hole, never to return.

Seeing the Invisible

As fantastic as they sound, black holes are commonplace throughout the universe; and scientists have much evidence that black holes exist even though they cannot be seen directly.

McClintock and Narayan studied a well-known black hole system called GRS 1915+105, in the constellation Aquila (The Eagle) about 35,000 light-years from Earth. The black hole itself is invisible, but matter pouring into it is exceptionally visible.

GRS 1915+105 is a two-star system. Gas from a "normal" star spills towards the black hole, lured by gravity. The gas spirals into the black hole like water down a drain: It doesn't just fall in all at once but first swirls around the black hole, forming a reservoir of matter called an accretion disk.

Gas in the accretion disk gets quite hot and emits light across a wide range of wavelengths. The inner part of the accretion disk, closest to the black hole, can be particularly bright in X-rays. Not all black holes are in two-star systems, but those that are can reveal themselves this way whenever they snack on gas from a companion star. Isolated black holes, on the other hand, only feed on the thin gas of interstellar space and are therefore very difficult to detect.

On the Edge of a Black Hole

Image/animation left: The faster a black hole spins, the closer matter can orbit safely around it. Click image to view animation (4.3 Mb -- no audio). Credit: NASA/GSFC

The science team, which includes Rebecca Shafee, McClintock's graduate student from Harvard University, focused on the accretion disk of GRS 1915+105 with the NASA's Rossi X-ray Timing Explorer. They concentrated on the inner edge of this disk, a region called the innermost stable circular orbit.

This region, Shafee said, is about 30 kilometers from the black hole center in GRS 1915+105, which is remarkably close. At an innermost stable orbit, gas can hover around the black hole indefinitely unless it is pushed. In between this region and the event horizon is somewhat of a no-man's land. Once matter gets bumped out of the innermost stable orbit, it falls toward the event horizon in less than a millisecond and is gone.

The distance from the innermost stable orbit to the black hole center depends on black hole spin. The faster the spin, the closer that matter can orbit "safely." This is a special property based on Einstein's theory of general relativity.

What McClintock and Narayan have done is accurately measure, for the first time, the distance of the innermost stable orbit to the black hole center. The measurement is based in part on the spectral analysis of the X-ray light. Simply put, when a black hole is spinning very fast, matter can orbit rather close to the event horizon; and as matter gets closer and closer to the black hole, the light it emits gets brighter and brighter. This information---brightness, temperature, velocity---is all encoded in the X-ray light.

From ground-based optical and radio observations, the scientists could determine the mass of the black hole, the angle of the tilt of the accretion disk in relation to us, and its distance from Earth. All these measurements feed into the equation to measure how fast a black hole is spinning.

What Exactly is Spinning, Again?

"The black hole spin frequency we measured is the rate at which spacetime is spinning, or is being dragged, right at the black hole event horizon," said Narayan.

This black hole is about 14 solar masses. This would imply, based on Einstein's math, that the event horizon is at a radius of 42 kilometers assuming no spin or at 21 kilometers assuming maximum spin. In reality, it is in between, at 25 kilometers from the black hole center.

At the point of the event horizon, the black hole is spinning at over 950 times per second. Place a flag here, and that flag would whip around the 157-kilometer track at a speed of 149,000 kilometers per second, or 50 percent the speed of light.

According to theory, the absolute maximum rate at which this black hole could possibly spin-essentially the speed of light-is 1,150 times per second. This means that the black hole is spinning at 98 percent its maximum value. (Note, this is not based on a simple calculation of 950/1150.)

The Big Picture

This clocking technique can be applied to any stellar-size black hole, and the team plans to measure the spins of about a dozen other well-studied systems. This would have broad implications for other topics in astrophysics, including understand such mysteries as black hole jets, gamma-ray bursts and gravitational waves.

From flying cosmic carpets moving nearly at warp speed to solving major mysteries in astronomy, it's clear we're in for quite a ride in the years to come.

Christopher Wanjek
NASA Goddard Space Flight Center