sherpa.astro.ui.eqwidth(src, combo, id=None, lo=None, hi=None, bkg_id=None, error=False, params=None, otherids=(), niter=1000, covar_matrix=None)

Calculate the equivalent width of an emission or absorption line.

The equivalent width 1 is calculated in the selected units for the data set (which can be retrieved with get_analysis).

Changed in version 4.10.1: The error parameter was added which controls whether the return value is a scalar (the calculated equivalent width), when set to False, or the median value, error limits, and ancillary values.

  • src – The continuum model (this may contain multiple components).

  • combo – The continuum plus line (absorption or emission) model.

  • lo (optional) – The lower limit for the calculation (the units are set by set_analysis for the data set). The default value (None) means that the lower range of the data set is used.

  • hi (optional) – The upper limit for the calculation (the units are set by set_analysis for the data set). The default value (None) means that the upper range of the data set is used.

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

  • bkg_id (int or string, optional) – The identifier of the background component to use. This should only be set when the line to be measured is in the background model.

  • error (bool, optional) – The parameter indicates whether the errors are to be calculated or not. The default value is False

  • params (2D array, optional) – The default is None, in which case get_draws shall be called. The user can input the parameter array (e.g. from running sample_flux).

  • otherids (sequence of integer or strings, optional) – Other data sets to use in the calculation.

  • niter (int, optional) – The number of draws to use. The default is 1000.

  • covar_matrix (2D array, optional) – The covariance matrix to use. If None then the result from get_covar_results().extra_output is used.


If error is False, then returns the equivalent width, otherwise the median, 1 sigma lower bound, 1 sigma upper bound, the parameters array, and the array of the equivalent width values used to determine the errors.

Return type


See also


Sum up the fitted model over a pass band.


Calculate the un-convolved model signal.


Return the default data set identifier.


Set the source model expression.





Set a source model (a powerlaw for the continuum and a gaussian for the line), fit it, and then evaluate the equivalent width of the line. The example assumes that this is a PHA data set, with an associated response, so that the analysis can be done in wavelength units.

>>> set_source(powlaw1d.cont + gauss1d.line)
>>> set_analysis('wavelength')
>>> fit()
>>> eqwidth(cont, cont+line)

The calculation is restricted to the range 20 to 20 Angstroms.

>>> eqwidth(cont, cont+line, lo=20, hi=24)

The calculation is done for the background model of data set 2, over the range 0.5 to 2 (the units of this are whatever the analysis setting for this data set id).

>>> set_bkg_source(2, const1d.flat + gauss1d.bline)
>>> eqwidth(flat, flat+bline, id=2, bkg_id=1, lo=0.5, hi=2)

With the error flag set to True, the return value is enhanced with extra information, such as the median and one-sigma ranges on the equivalent width:

>>> res = eqwidth(p1, p1 + g1, error=True)
>>> ewidth = res[0]  # the median equivalent width
>>> errlo = res[1]   # the one-sigma lower limit
>>> errhi = res[2]   # the one-sigma upper limit
>>> pars = res[3]    # the parameter values used
>>> ews = res[4]     # array of eq. width values

which can be used to display the probability density or cumulative distribution function of the equivalent widths:

>>> plot_pdf(ews)
>>> plot_cdf(ews)