Tomography is a technique which is similar to performing a CAT scan on an X-ray binary. Binaries emit radiation as they rotate around their center of mass at high speeds. The wavelength of the radiation contribution of a particular point on the system is Doppler shifted due to its radial velocity. This information is stored in the line profiles observed in an optical spectrum taken using a telescope. A time series of these line profiles can be used to recreate the emission regions of the system in velocity space. These velocity maps (or tomograms) can be helpful in identifying the nature of the outer accretion disk.
The figure on the left shows an Hα tomogram of the black hole binary Nova Muscae 1991 created using data obtained from the MagE instrument on the Magellan telescope in La Serena, Chile, in 2009. The trailed spectra is shown in the top-left panel; the Doppler map is on the
bottom-left; the modulation-amplitude map is on the bottom right; the predicted trail from the Doppler map is on the top right. The Doppler map shows the hot-spot where the accretion stream from the secondary star hits the accretion disk at phase ~0.8.