Artist's illustration of Cygnus X-1, the first black hole, which was discovered in 1972. We recently measured to 6 percent accuracy its distance (1.86 kpc) using the VLBA and its mass (14.8 solar masses) using extensive optical data. Chandra data reveal that the compact object is a near-extreme Kerr hole with a spin a/M > 0.95. Credit: Optical: DSS; Illustration: NASA/CXC/M.Weiss
Knowledge of black hole spin is essential for understanding such empirical issues as how gamma-ray bursts are powered and how black holes launch jets and other outflows that inject energy into the surrounding medium and affect structure formation on the scale of galaxies and even clusters of galaxies. During the past several years, we have established an accurate method for measuring the spins of stellar-mass black holes located in X-ray binary systems. This has allowed us to completely describe a dozen of these black holes by measuring both their spins and masses.
At the same time, given our deep knowledge of how these particular black holes behave, we now find ourselves at an exciting jumping-off place. For example, during this past year we published the first direct evidence for a relationship between jet power and black hole spin. Beyond astrophysics, our aspiration is to use secure measurements of black hole spin as a basis for making a compelling experimental test of the no-hair theorem.