High-mass X-ray Binaries

If the companion star in an X-ray binary system has a mass greater than 10 solar masses, the system is known as a high-mass X-ray binary (HMXB). In this case, the companion is generally an O or B type star which emits a stellar wind driven by radiation pressure.
Mass transfer to the neutron star or black hole does not proceed via Roche-lobe overflow into an accretion disk as is the case for a low-mass X-ray binary or a cataclysmic variable, but rather through the capture of this stellar wind directly onto the compact object. The potential energy of the wind is converted into X-rays which dominate the emission from the neutron star or black hole.
Although they are strong X-ray sources, it is also possible to observe a reasonable fraction of high-mass X-ray binaries at visual wavelengths, since their optical emission is dominated by the massive companion star. Most are found in the disk of the Galaxy and almost 2/3 of them are actually Be X-ray Binaries.

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