Model components are combined in the obvious algebraic way, with + seperating additive models, * separating multiplicative models, and parentheses to show which additive models the multiplicative models act on (syntactically, convolution and mixing models are treated as multiplicative models). The * need not be included next to parentheses, where it is redundant. Also, if only one additive model is being modified by one or more multiplicative models, the required brackets may be replaced by a *. In this case the additive model must be the last component in the grouping. Thus
is a valid model, where the M's signify multiplicative models and the A's additive models. One may also specify one additional nesting of parentheses, so that one or more overall multiplicative components are specified, along with the possiblity of having additive components outside the overall multiplicative grouping. Thus
is a valid model.
The old style syntax for entering models (versions 9.02 and earlier) is still supported, with the following restriction : any background components must be entered on the command line as the last components in the component list.
When XSPEC parses the model syntax, it organizes the model components into additive groups, each group containing additive components and the multiplicative components which modify them. In addition there is an overall additive group, which contains the overall multiplicative components and any additve components outside the overall multiplicative grouping. Therefore, if Aij is the ith additive model in the jth group, and similarly Mij is a multiplicative model, the total spectrum is given by
M10*M20*... * ( [(A11+A21+...)*M11*M21*...] + [(A12+A22+...)*...] + ... ) + A10 + A20 + ...
Each component may have one or more parameters that can be varied during the fit (see the newpar command writeup).
When using convolution models, the order in which they are applied in the additive group can be significant. For example, the two models
are not necessarily equivalent (here the C's represent convolution models). The way XSPEC handles the ordering of components is by first computing the spectrum for the additive components of a given additive group (A1+A2 in the above example). It then applies all multiplicative or convolution components in the additive group from right to left, in the order they appear in the model formula. For example, in the model
The multiplicative and convolution components would be applied in the order M4, C3, M2, C1. Overall multiplicative and convolution components are handled similiarly. Any mixing models in an additive group are applied last, after any convolution or multiplicative models.