The model spectrum, f(E), is calculated within XSPEC using the energy ranges defined by the response file :
and is in units of photons/cm^2/s. XSPEC allows the construction of composite models consisting of additive components (e.g., power-laws, blackbodys, and so forth), and multiplicative components, which modify additive components by an energy-dependent factor (e.g., photoelectric absorption, edges, ...). Models can be defined in algebraic notation. For example, the following expression:
defines an absorbed blackbody (phabs (bbody)) added to a power-law (power). The result then is modified by another absorption component (phabs).
For a more detailed explanation of models, see Chapter 6.