X-ray Binaries are systems composed of a compact object (a black hole or a neutron star) accreting matter from a companion star. They are classified depending on the mass of the companion star: while Low-Mass X-ray Binaries usually contain a red giant or a Main Sequence star, High-Mass X-ray Binaries usually contain an OB supergiant or a Wolf Rayet star. 

During the accretion of the stellar matter towards the compact object, the conservation of the angular momentum generates an accretion disk around the latter. The thermal photons coming from the disk are then inversely Comptonized at the disk's surface and gain energy from the high-energy electrons, resulting in an intense emission in X-rays. We then collect the photons using X-ray telescopes to analyse the different emission contributions from the disk using X-ray spectra. We can model the accretion disk as an optically thick, geometrically thin disk emitting like a black body (from Shakura and Sunyaev solutions) and the Comptonized hot medium as an optically thin, geometrically thick corona, emitting like a power law.

The analysis of the spectra is necessary to understand the emission mechanisms from the different contributions from the disk, alongside the accretion/ejection processes in extreme environments around compact objects such as black holes.