For this case, we will use a linearly varying background with different time intervals for the global energy range and for each of four user-selectable energy ranges. We first set the order of the polynomials to be used to fit the background in the different time intervals we will select by typing the following:

SPEX> back_order,1,1,1,1,1

Note that the first "1" applies to the global energy range and the other four apply to each of the four user-selectable energy bins. Different background time intervals can be used for different energy bins with the parameter, use_band. The default value for use_band is -1 and this means that the background intervals apply to the global energy range. There are currently four different energy bands defined by the array ENERGY_BANDS, the same array set above for displaying the time history.

The background intervals can be set independently for the global energy range and for each of the four user-selectable energy bands (that may or may not cover the full global energy range of the selected data). We have already seen above how to select the background intervals for the global energy range by typing "background" and selecting the start and end of each interval manually. This MUST be done first with use_band set to the default value of -1. Then the background intervals are selected for each of the four energy bands in turn by issuing the following commands, where the energy band number (0, 1, 2, or 3) is substituted for i:

SPEX> use_band = i
 

Note the equals sign instead of the comma since this is interpreted by SPEX as an IDL command and not a SPEX command.

The background intervals are then set for each energy band by typing:

SPEX> background
 

On the pop-up window, click on the "Continue by Selecting from Plot Window" button. Then move the mouse over the window with the active display ("Time History Window"). Push the left mouse button to define each side of the time intervals to be used for background determinations in that particular energy range. Note that the left edge will automatically revert to the data interval to the left of where you select; the right edge will revert to the next edge of a data interval to the right of where you select. Also, be careful that you don't click too fast. Wait after each click to see the vertical line appear on the plot and see the message appear in the IDL output log window.

Once you have selected all the intervals for background determination for that particular energy interval, move the cursor out of the Time History Window and click "DONE."

The program then computes the background as a function of time for that particular energy band and subtracts it from all the time intervals. A new time history is automatically plotted with the background subtracted.

Note that I chose the first and last 1-minute intervals as background for the two lowest energy bands. For the 25 - 50 keV band, I chose intervals 3 and 9; for the 50 - 100 keV band, I chose intervals 6 and 8.

After you have selected the background and subtracted it for the global energy range and for all four energy bands, the Time History Window should look something like this:

You can now go on to select the time interval(s) for analysis as before and determine the best-fit spectra with the new background estimates.

This page last updated: June 27, 2011