Light From 30 Miles Above a Black Hole Reveals Spinning - April 2001
by Christopher Wanjek
As if black holes couldn't be more menacing, astronomers have further evidence that at least some of them spin about like whirlpools, wrapping up the fabric of space with them.
Dr. Tod Strohmayer of NASA's Goddard Space Flight Center in Greenbelt, Md., has studied one such black hole system with the Rossi X-ray Timing Explorer and found unique patterns in the X-ray radiation about 30 miles from the black hole that have previously only been seen in spinning neutron stars. With these new parameters, he could verify that a black hole, like a neutron star, can spin.
The observation also challenges theories about neutron star radiation. Strohmayer presents his findings today at the April 2001 Meeting of the American Physical Society in Washington, D.C. Computer animation illustrating the discovery will be broadcast on NASA TV at noon EDT today.
"Almost every kind of object in space spins, such as planets, stars, galaxies," said Strohmayer. "With black holes, it's much harder to directly see that they are spinning, because they don't have a solid surface that you can watch spin around. We can, however, see the light emitted from matter plunging into the black hole. The matter, cannibalized from the black holes binary companion star, whips frantically around the black hole before it is lost forever."
The black hole that Strohmayer observed is the stellar variety, which is formed from a collapsed star. When stars at least 10 times more massive than our Sun exhaust their fuel supply, they no longer have the energy to support their tremendous bulk. These stars explode their outer shell of gas in an event called a supernova. The remaining bulk, still several times more massive than the Sun, collapses into a single point of infinite density, called a singularity.
Neutron stars form through a similar process, only from a slightly less massive star that collapses into a dense chunk as heavy as the Sun yet only ten miles across. Astronomers can "see" neutron stars and black holes by virtue of the hot, glowing gas -- often from a neighboring star -- that orbits these objects at near light speed.
The Rossi Explorer has long recorded a certain type of X-ray flickering from neutron stars called quasiperiodic oscillations, caused by hot gas dancing around the neutron star in a lively orbit. Astronomers think that these oscillations are produced by motions of matter very near the innermost stable orbit -- the closest orbit a blob of gas can maintain before falling pell-mell into the central object.
Many astronomers believe the oscillation frequency is related to the mass of the central object and that one can estimate the location of the innermost stable orbit from this frequency.
Strohmayer's target was GRO J1655-40, a microquasar 10,000 light years from earth. (The black hole menace continues: A microquasar is a specific type of black hole with jets of high-speed particles shooting perpendicularly from the plane of matter that orbits it.) Strohmayer observed two strong oscillations, a previously detected one at about 300 Hz and a newly detected one at 450 Hz. (A hertz, Hz, is a unit of frequency equal to one cycle per second; the electricity in household appliances oscillates at 60 Hz.)
GRO J1655-40 is a well-studied system, and the black hole mass has been established at 7 solar masses from earlier optical observations. Applying Einstein's theory of general relativity, this mass implies an innermost stable orbit of about 40 miles if the black hole were not spinning.
A QPO at 450 Hz, however, implies an innermost stable orbit with a radius less than or equal to about 30 miles. The only way that gas can maintain a stable orbit that close to the black hole, Strohmayer said, is if the black hole is spinning.
"A spinning black hole modifies the fabric of spacetime near it," said Strohmayer. "The spinning allows matter to orbit at a closer distance than if it were not spinning, and the closer matter can get the faster it can orbit. For GRO J1655-40 we can now say that the only way for it to produce the 450 Hz oscillation is if it is spinning."
The first detected oscillation, at 300 Hz, was coincidentally in tune with a 64-km innermost stable orbit. It was the detection of the higher frequency oscillation at 450 Hz that implied spin.
At least three earlier observations of black holes have provided evidence of s pin. The Rossi Explorer captured lower-frequency oscillations from black holes that could be explained as the movement of spacetime around a black hole, called Lense-Thirring precession, which implies that the black hole is spinning. The Japan-U.S. ASCA X-ray satellite found distorted X-ray light from quasars that could be explained as matter orbiting close to a spinning supermassive black hole.
Strohmayer's finding also marks the first detection of paired oscillations from a black hole. Neutron stars often exhibit paired oscillations, and this is thought to be a result of radiation coming from the solid neutron star surface. Strohmayer's detection of paired oscillations from an object thought to have no solid surface, therefore, challenges these important theories of how neutron stars produce these oscillations.
The spin of a black hole, incidentally, would be caused by the angular momentum of the star that formed it, Strohmayer said, particularly if that progenitor is a spinning neutron star, which can theoretically collapse into black holes if they accrete enough matter.
Compared to a non-spinning black hole, a spinning black hole has a smaller event horizon, that theoretical boundary around a black hole beyond which nothing can escape. Proposed X-ray space-science missions such as Constellation-X would be able to study black hole spin in greater detail, observing how a spinning black hole more strongly warps spacetime and stretches light emitted by hot gas, a test of general relativity.
The Rossi Explorer, launched in December 1995, is a unique X-ray satellite designed to record rapidly fluctuating objects, such as pulsars and neutron stars, which can spin over a thousand times per second. For more information about Rossi, refer to http://heasarc.gsfc.nasa.gov/docs/xte/learning_center/.