X-ray binary
= X-ray binary
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An X-ray binary has a compact <mass-gaining star> or <black hole> accreting from a companion, so that much of its emitted power appears as <X-rays>. Gravitational binding energy supplies an <accretion luminosity> of order $GM\dot M/R$ when there is a surface or an appropriate inner disk radius. The <donor star> may feed an <accretion disk> through <Roche-lobe overflow> or a <stellar wind>.