Algorithm

 compute the background lightcurve of the problem event file applying
 the following selection expression

 CCDNR==9 AND (XDSP_CORR < -3.E-4 OR XDSP_CORR > 3.E-4)

 From the lightcurve, calculate the fraction of the total time for
 each of the pre-defined levels of background.

 Compute the background template spectra by weighting each of the
 template spectra by the corresponding factor calculated from the
 ligthcurve.