How does gas orbiting a black hole in an accretion disk lose angular momentum so that it can spiral down toward the black hole, heating up and emitting X-rays as it falls inward?

X-ray binary stars, like Cygnus X-1, contain a black hole or neutron star in a close orbit with a normal companion star. The tidal pull of the compact star strips gas off the normal star, and as this gas falls in toward the compact star it is heated up to millions of Kelvins. This hot, accreting gas emitts a prodigious amount of X-ray radiation, making these systems the brightest X-ray stars in the sky. But how this gas looses angular momentum and continues to fall in through the disk is a perplexing question.