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ASTRONOMY
By definition, black holes cannot be observed directly. Their effects on nearby light and matter, however, have been seen so often that we are forced to conclude that black holes really do exist. To some extent, we can even understand how they are created. When a massive star dies, it leaves behind a "compact object." If the mass of the residual compact object is more than about three times the mass of the Sun (the so-called Oppenheimer-Volkov limit), then nothing can hold back gravity and the residual core of the death star becomes a black hole (1).
In addition to their mass, black holes are also characterized by their spin. The existence of radio pulsars demonstrates that stellar remnants are spinning rapidly when formed, so it seems likely that all stellar-mass black holes are spinning, at least initially. The rotation (6) of black holes is quantified by the dimensionless spin parameter a* = Jc/GM^sup 2^, where Jand M are the angular momentum and mass of the black hole, respectively, c is the velocity of light, and G is the gravitational constant. Gas flowing toward the black hole slowly reaches the inner part of the accretion disk up to the innermost stable circular orbit (ISCO). The crucial difference between a spinning black hole (called a Kerr black hole) and one that is not spinning (a Schwarzschild black hole) is that rotation allows the ISCO to move inward in case of corotation (7). This allows the accretion disk to reach closer into the black hole, which in turn permits higher orbital frequencies and higher orbital velocities in the disk. In recent years, there have been growing indications that black holes may indeed be spinning. These mainly come from two new observational constraints that shed light on the accretion flow close to the black hole.
In the immediate vicinity of black holes, millisecond variations of x-ray emission are expected. This has been observed in the form of high-frequency quasiperiodic oscillations (QPOs) in the x-ray power density spectra of stellar-mass black holes (8, 9). Usually, these QPOs are detected when the soft x-ray emission is very high, which is when the accretion disk is very close to the black hole. How close is a matter of debate,...