Define the form of the theoretical model to be fit to the data.
XSPEC> model <component1> <component2> ... <componentN>
where <componentJ> is one of the models known to XSPEC. Descriptions of these models may be accessed using the help model command. The models are divided into two types: additive and multiplicative models. Additive models are those directly associated with emissivity, such as power laws, thermal models, emission lines, etc. The net effect of two independent additive models is just the sum of their individual emissivities. Multiplicative models, on the other hand, do not directly produce photons, but instead modify (by an energy-dependent multiplicative parameter) the spectrum produced by one or more additive components. Examples of multiplicative models are photoelectric absorption models, edges, absorption lines, etc.
A list of all the currently-installed models is given in response to the component ?.