If the number of net counts for a source is less than 
it is
impractical to fit the data in all 34 energy bits. However, if the
flux is required, then a fit must be performed first. This
can be done by setting the hidden parameter rebin=yes, which bins
the data into a single bin before fitting for the normalization only.
The user must specify all the parameters in the
chosen model except the normalization.
Fit will then calculate the normalization of the model and
xflux can
be used to determine the flux and luminosity of the source from
the best fit model.
xs> fit rebin=yes