Used with spex_bk_sm_width (see Step 2 below)
For each energy bin, the background time profile is computed by multiplying the high-energy profile by the ratio of the rate in that energy to the high-energy background profile in the times selected for that energy. The result will be smoothed using the spex_bk_sm_width (number of points) specified. When you use this option, the polynomial order value is ignored for all bands except the high-energy band.
More specifically:
1. The background profile is computed by performing a fit of the data in the highest energy band you selected to a polynomial of the order you selected.
2. This profile is smoothed using the smoothing width specified (the profile is convolved with a Savitzky-Golay smoothing filter of the width selected).
3. For each data energy bin (the raw bins, not the broader energy bands you have selected), the data in the time intervals selected for the energy band containing this bin are averaged and divided by the average of the high-energy smoothed profile in those times.
4. The background time profile for this energy bin is computed by multiplying the high-energy smoothed profile by that ratio.
5. The errors on the background are the square root of the expected counts in each bin.