X-ray binaries owe their prominence to one of the most efficient energy release mechanisms known: accretion onto a compact object. In the accretion process gas from the atmosphere of the less dense companion is accelerated toward the extremely dense compact star. Because the stars are orbiting each other, the gas cannot fall directly onto the compact star, but forms an encircling disk of material. As matter spirals though this accretion disk, it is accelerated and heated, causing it to radiate at ever increasing energies. The energy is released over essentially the entire electromagnetic spectrum, with each part of the spectrum revealing information, often time-variable, characteristic of particular segments of the system. The physics behind these processes is complex, involving not only dynamics of gases under gravity, but also magnetic fields, radiative transfer, and poorly-understood viscous and other dissipational effects without which the essential inward transfer of matter through the disk would be impossible.

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