fit_bkg

sherpa.astro.ui.fit_bkg(id: IdType | None = None, *otherids: IdType, **kwargs) → None

Fit a model to one or more background PHA data sets.

Fit only the background components of PHA data sets. This can be used to find the best-fit background parameters, which can then be frozen before fitting the data, or to ensure that these parameters are well defined before performing a simultaneous source and background fit.

Changed in version 4.17.1: The parameter record_steps was added to keep parameter values of each iteration with the fit results.

Changed in version 4.17.0: The outfile parameter can now be sent a Path object or a file handle instead of a string.

Parameters:

  • id (int or str, optional) – The data set that provides the background data. If not given then all data sets with an associated background model are fit simultaneously.

  • *otherids (sequence of int or str, optional) – Other data sets to use in the calculation.

  • outfile (str, Path, IO object, or None, optional) – If set, then the fit results will be written to a file with this name. The file contains the per-iteration fit results.

  • clobber (bool, optional) – This flag controls whether an existing file can be overwritten (True) or if it raises an exception (False, the default setting). This is only used if outfile is set to a string or Path object.

  • record_steps (bool, optional) – If True, then the parameter values and statistic value are recorded at each iteration in an array in the FitResults object that you can obtain with get_fit_results.

Raises:

sherpa.utils.err.FitErr – If filename already exists and clobber is False.

See also

calc_bkg_stat, conf, contour_fit, covar, fit, freeze, get_fit_results, plot_fit, image_fit, set_stat, set_method, set_method_opt, set_bkg_full_model, set_bkg_model, set_full_model, set_iter_method, set_model, show_bkg_source, show_bkg_model, show_fit, thaw

Notes

This is only for PHA data sets where the background is being modelled, rather than subtracted from the data.

The screen output from the fit can be controlled with the SherpaVerbosity context manager, as shown in the examples.

If outfile is sent a file handle then it is not closed by this routine.

Examples

Simultaneously fit all background data sets with models and then store the results in the variable fres:

>>> fit_bkg()
>>> fres = get_fit_results()

Fit the background for data sets 1 and 2, then do a simultaneous fit to the source and background data sets:

>>> fit_bkg(1, 2)
>>> fit(1, 2)

Store the per-iteration values in a StringIO object and extract the data into the variable txt (this avoids the need to create a file):

>>> from io import StringIO
>>> out = StringIO()
>>> fit_bkg(outfile=out)
>>> txt = out.getvalue()

The messages from fit_bkg use the standard Sherpa logging infrastructure, and so can be ignored by using SherpaVerbosity:

>>> from sherpa.utils.logging import SherpaVerbosity
>>> with SherpaVerbosity("WARN"):
...     fit_bkg()
...
>>> fres = get_fit_results()