If the orbit is small enough the normal star will evolve and expand to a red giant until it fills its Roche Lobe. At this point it will "spill" over through the inner Lagrange point, dumping mass into the Roche lobe of the black hole. This gas will form an accretion disk around the black hole, and as the gas spirals in it will heat up and emit X-rays.

Here at NCSU we have developed numerical models of X-ray binaries. Recent work includes 3D simulations and models of eccentric X-ray binaries.