# set_bkg_model¶

sherpa.astro.ui.set_bkg_model(id, model=None, bkg_id=None)

Set the background model expression for a PHA data set.

The background emission can be fit by a model, defined by the set_bkg_model call, rather than subtracted from the data. If the background is subtracted then the background model is ignored when fitting the data.

Parameters: id (int or str, optional) – The data set containing the source expression. If not given then the default identifier is used, as returned by get_default_id. model (str or sherpa.models.Model object) – This defines the model used to fit the data. It can be a Python expression or a string version of it. bkg_id (int or str, optional) – The identifier for the background of the data set, in cases where multiple backgrounds are provided.

delete_model()
Delete the model expression from a data set.
fit()
Fit one or more data sets.
integrate1d()
Integrate 1D source expressions.
set_model()
Set the model expression for a data set.
set_bkg_full_model()
Define the convolved background model expression for a PHA data set.
show_bkg_model()
Display the background model expression for a data set.

Notes

The function does not follow the normal Python standards for parameter use, since it is designed for easy interactive use. When called with a single un-named argument, it is taken to be the model parameter. If given two un-named arguments, then they are interpreted as the id and model parameters, respectively.

The emission defined by the background model expression is included in the fit to the source dataset, scaling by exposure time and area size (given by the ratio of the background to source BACKSCAL values). That is, if src_model and bkg_model represent the source and background model expressions set by calls to set_model and set_bkg_model respectively, the source data is fit by:

src_model + scale * bkg_model


where scale is the scaling factor.

PHA data sets will automatically apply the instrumental response (ARF and RMF) to the background expression. For some cases this is not useful - for example, when different responses should be applied to different model components - in which case set_bkg_full_model should be used instead.

Examples

The background is model by a gaussian line (gauss1d model component called bline) together with an absorbed polynomial (the bgnd component). The absorbing component (gal) is also used in the source expression.

>>> set_model(xsphabs.gal*powlaw1d.pl)
>>> set_bkg_model(gauss1d.bline + gal*polynom1d.bgnd)


In this example, the default data set has two background estimates, so models are set for both components. The same model is applied to both, except that the relative normalisations are allowed to vary (by inclusion of the scale component).

>>> bmodel = xsphabs.gabs * powlaw1d.pl
>>> set_bkg_model(2, bmodel)
>>> set_bkg_model(2, bmodel * const1d.scale, bkg_id=2)