A very simple way of fitting with user-defined models is available for
a particular class of models. These are models that can be defined by
a grid of spectra, with the elements of the grid covering the range of
values of the parameters of the model. For instance, for a
one-parameter model, a set of model spectra can be tabulated for
different values of the parameter (P1, P2, P3, etc.) The correct model
spectra for a value P then is calculated by interpolation on the
grid. The generalisation to more parameters works in the obvious way.
As with standard models, the spectra should be in terms of
flux-per-bin and not flux-per-keV. Any set of energy bins can be used,
and XSPEC will interpolate the model spectra onto the appropriate
energy bins for the detectors in use. It is therefore a good idea to
choose energy bins such that the spectrum is well-sampled over the range of
interest. The file structure for these models is a FITS format and is
described in the
OGIP memo OGIP/92-009,
also available by
anonymous ftp.