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Figure 7.34. The optical spectrum of the LMXB Sco X-1. Adapted from Steeghs & Casares (2002).

typically adopt an observationally easier (if somewhat less informative) division between "low-mass" and "high-mass" X-ray binaries. It is important to note that the mass referred to here is NOT that of the compact object - rather it is the mass (in practice, spectral type or luminosity class) of the donor star.

For low-mass XRBs (LMXBs), the spectrum is typically dominated by the accretion disk itself, rather than by the low-mass (and thus typically low-luminosity) donor star. Typical features are broad emission lines of H, He I, and He ii. We also often see high ionization states of metals (Figure 7.34). Radial velocity measurements can give mass-function measurements, and line profiles can even yield Doppler tomograms like those obtained for CVs. The IR spectra also show similar features. These features are often

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