Besides the instrumental background a constant source background * sbg* can
be added (units [counts/Å]) accounting for eventual continuum radiation or
unresolved weak lines (''Pseudo continuum'') originating from the source.
If the switch * sbgit* (on the same line of the parameter file) is activated,
then the given
value for * sbg* is ignored, and the source background will be determined by
by an iteration of median values, applied to the part of the spectrum under
consideration (see below).

The third value on this input line, the * bgsmooth* switch, decides whether
the instrumental background will be smoothed, i.e., that a polynomial of
2 order is used to model the instrumental background (a polynomial
of lower order is used in the case that the 2 order polynomial
fails). Only the part of the spectrum under consideration is smoothed in this
way.

More refined methods to estimate the source continuum are provided (see Ness et al.
2003, A&A **407**, 347 and 2004, A&A **427**, 667).
A
method is programmed using the model parameters given by start values in order
to generate a model spectrum. This is added to a source background value to be
iterated by minimizing . This method is a good approach in cases when
line wings are broad (thus, too many bins belong to emission lines) and when
no lines features in the spectrum are excluded from the fit. With the fourth
number on the input line in the parameter file this procedure can be activated.
The last entry in that line () is a value used for a refined median
method. Since the median has to rely on that at least half
of all bins have to belong to the background, too crowded line regions can be
problematic. This can be solved by removing all bins which obviously belong to
lines from the list of bins used to calculate the median value. In this way
median values are iteratively calculated after removal of all bins with count
values higher than
. Small values
of will more critically remove high-count bins resulting in lower
source background values. Usage of this parameter represents a parameterized
choice of source continuum values by eye.