An X-ray binary is a system composed of a compact object (a white dwarf, a neutron star or a black hole), accreting matter from its companion star. The spectral type of the companion is crucial in the mass transfer mechanism, and these systems are therefore separated into high mass X-ray binaries (HMXB, spectral type OB), and low mass X-ray binaries (LMXB, A type or later).
- High Mass X-ray Binaries (HMXB)
In these systems the optical star is an OB star. We can also distinguish between the different subgroups based on the luminosity class of the optical star, as this characteristic also plays an important role in how the matter is transferred. For example, if the optical star is a supergiant its strong radiation driven wind boosts the accretion onto the compact object, forming a supergiant X-ray binary system (SGXB).
If the optical star is a main sequence or giant star, it may exhibit a decretion disk where matter accumulates and eventually falls down onto the compact object. These stars (mostly B’s) are classified as Be stars, because the decretion disk is also responsible for the emergence of forbidden emission lines. Such a system is known as a Be X-ray binary system (BeXB).
γ-Cassiopeae is a special case in the BeXB. Its optical star is indeed a Be, but the X-ray spectrum shows completely different behavior in comparison to normal BeXB. The discovery of 6 similar sources motivated a new HMXB class definition, the γ-Cassiopeae-like stars. The possibility of a non-accretion scenario has not been ruled out in this case, because the interaction of the stellar surface and the magnetic field of the decretion disk is able to explain some aspects of the emission observed, and γ-Cassiopeae is the only one of these sources confirmed as a binary. It is also possible that the spectral energy distribution of these objects is produced by the accretion of matter onto a white dwarf rather than a neutron star. However, the issue is still open. A conference has been organised to attack the gamma Cas problem from an interdisciplinary perspective.
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