Calculate the flux of the current model between certain limits.
XSPEC> flux [<lowEnergy> [<hiEnergy>]]
where <lowEnergy> and <hiEnergy> are the values over
which the flux is calculated. Initial default values are 2 to 10 keV.
The flux is given in units of photons/cm 
/s and ergs/cm /s. The
energy range must be contained by the range covered by the current
data sets (which determine the range over which the model is
evaluated). Values outside this range will be reset automatically to
the extremes. Note that the energy values are two separate arguments,
and are NOT connected by a dash.