4. Classification of X-Ray Binaries

X-Ray binaries are conventionally divided into two classes: the low-mass X-ray binaries (LMXB) and the high-mass X-ray binaries (HMXB) (Bradt and McClintock (1983)). Bradt and McClintock based their classification scheme on the ratio of X-ray to optical luminosity. For a HMXB $${L_{\rm x}(2-10{\rm\ keV})\over L_{\rm opt}(3000-7000\rm\ \AA)} \mathrel{\hbox{\rlap{\hbox{\lower4pt\hbox{$\sim$}}}\hbox{$>$}}}10$$
while for a LMXB this ratio is greater than about 10. This division based on optical and X-ray luminosities coincides with a division based on the mass of the companion. This is because if the companion is an early type massive star it will dominate the optical luminosity. If the companion is a low mass star the optical luminosity will be dominated by the accretion disk.