What defines X-ray binaries?

X-ray binaries are defined as binary star systems in which one star, often a neutron star or black hole, is in close proximity to a second star and exhibits intense X-ray emissions. This intense radiation is produced when the companion star's material is pulled towards the black hole or neutron star due to its strong gravitational field. As the gas from the companion star spirals into the compact object, it heats up to extremely high temperatures, resulting in the emission of X-rays.

The characteristics of X-ray binaries highlight the interactions and complexities between the two stars in the system. In contrast, options that mention both stars being red giants misrepresent the nature of the system, as red giants are typically not the end stages of such binary systems. Additionally, a binary system emitting no radiation does not align with the defining features of X-ray binaries, which are characterized by their significant radiation due to the accretion process. Lastly, a system with planets orbiting a single star completely diverges from the definition of a binary star system, as X-ray binaries specifically involve two stars, not planets. Therefore, the accurate description of an X-ray binary is the one highlighting the intense X-ray emissions from one of its stars.