Measuring Spinning Black Holes

March 24, 1997

UPDATE April 10, 1997: This research has been accepted for publication in The Astrophysical Journal Letters, and a copy of the paper may be obtained at http://xxx.lanl.gov/abs/astro-ph/?9704072.

Using data from several NASA satellites, Dr. Shuang Nan Zhang of the Universities Space Research Association at NASA/Marshall Space Flight Center and collaborators Dr. Wei Cui of the Massachusetts Institue of Technology and Dr. Wan Chen from the University of Maryland at College Park and NASA/Goddard Space Flight Center have measured the spins of several black holes, by accurately measuring the size of the last stable orbit of material around the black hole.

In the past several years, evidence has been accumulated that there are indeed such things as black holes in the Universe. Not only do they exist in the center of many galaxies with masses between millions and billions times the Sun, such as in this Hubble Space Telescope photo of the center of the galaxy M87, about a dozen of them have been found fairly nearby in our own Galaxy with masses between 3 and 30 times that of the Sun.

These lighter ones, usually called stellar mass black holes, all reside in so-called "binary systems," with the black hole's companion star being rather ordinary. By examining the relative movements of both objects in a binary system, one can infer the mass of the invisible object - the black hole candidate. The black hole candidate is believed to be a real black hole, when the amount of mass more than three times that of the Sun is found to be concentrated within a radius of about 10 kilometers, since theoretically no other objects may have such a high degree of mass concentration.

With a fairly robust way to measure the mass of a black hole, the next problem is to study the other properties such as the charge and the spin. Currently astronomers have not figured out on how to observe the black hole charge. "Tremendous progress, however, has been made in the study of the black hole spin," said Dr. Zhang. "Relativity theory tells us that there should be a last-stable-orbit around the black hole. Material inside this orbit cannot survive and is consumed by the black hole. The size of this orbit is related to the spin of the black hole, so by looking at the material that occupies this orbit, and measuring its extent, we can learn about how fast the black hole is spinning."

Using this technique, Dr. Zhang and his colleagues have measured the spins of several black holes. In their recently submitted publication, Zhang and colleagues describe that two of the black holes they studied are rapidly spinning, while others are not. "Coincidentally these two rapidly spinning black holes also occasionally eject relativistic jets," said Zhang. A relativistic jet is a stream of high-speed material ejected from the black hole region at nearly the speed of light. Their continued observations of black holes promise to provide more insight into these fascinating objects.

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