The Milky Way contains a few hundred million black holes, which were formed by the collapse of very massive stars. Each of these stellar black holes weighs about 10 times as much as our sun. A very few of these black holes are closely orbited by an ordinary star that is slowly bleeding matter onto the black hole. As this gas falls toward the black hole, it is heated by strong gravity and friction to a temperature of millions of degrees thereby producing a brilliant source of X-rays that is brighter than 10,000 suns. These black-hole X-ray sources are easily observed throughout the Milky Way, and in nearby galaxies as well, using orbiting X-ray observatories. Very remarkably, a black hole is fully described by just two numbers that specify its mass and how fast it is spinning; we know of nothing else this simple except for an elementary particle such as an electron. During the past 30 years, we and other observers have determined the masses of 21 of these black holes by measuring the velocities of their companion stars. Recently, we have obtained estimates of the spins -- and hence a complete description -- of three of these 21 black holes by fitting their broadband X-ray spectra. Our group is currently focused on applying this method to additional black holes, the spin results for which will be compared against determinations based on observed broadened Fe emission lines (in the same sources) attributed to strong relativity effects.

• Constellation-X
• The Near-Extreme Kerr Black Hole GRS 1915+105
• Black Hole Science with Constellation-X

HEA: J.E. McClintock, M. R. Garcia, J. C. Lee, D. Steeghs, M. A. P. Torres, S. Vrtilek
TA: R. Narayan