XMM-Newton Science Analysis System: User Guide

4.8.3 Generating source and background spectra in one go

The following section shows an alternative approach to generate source and background spectra in a single step (optionally together with related response matrices - see § 4.8.2): the xmmselect product selection "OGIP Spectral Products" starts the meta-task especget which is a one-stop task producing all the files necessary for the spectral fitting of an XMM-Newton source. especget runs the tasks evselect, arfgen and rmfgen. It also calculates the size of the source and background areas by calling backscale. The end result is a set of files which can be used directly in a spectral fitting programme like e.g. Xspec.

The interactive steps needed during the "OGIP Spectral Products" approach (assuming that an EPIC image was created with xmmselect) are listed below:

1. In the ds9 window, create a region for the source of interest. Click once on the ds9 image and a region circle (default shape) will appear. Click on the region circle and the region will be activated, allowing the region to be moved and its size to be changed. Having created, placed, and sized the region appropriate for the source, one needs to define a region from which to extract the background: this is done in a similar way as before for the source region, but now this second region must be placed in a source-free area (see recommendations on where to place the background region given earlier in this section). Click on the background region and modify its position and size. Via the ds9 "Region" $\rightarrow$ "Properties" menu the region types must be defined as "Source" for the source and "Background" for the background region, respectively.
2. Start the spectral product generation by clicking on the "OGIP Spectral Products" product selection in xmmselect (cf. figure 20). The task eregionanalyse performs a source region optimization and the optimized source region is shown in the ds9 window, the proposed region parameters are given in a popped-up window and the user is asked to confirm if the optimized or the original region should be used for further analysis (or if the spectral product generation is aborted at this stage).
3. If a source region is accepted, the especget GUI appears and shows the corresponding spatial selections for the source (parameter srcexp) and background (parameter backexp) regions (figure 26). The user might want to modify the stem for the output filenames (parameter filestem on "filenames" related parameter page). For extended sources, the parameter extendedsource (on the "effects" related parameter pages) should be set to true. Pressing the "Run" button of the GUI starts the generation of source and background spectra (and response matrices). The meta-task especget finishes the processing displaying the generated source spectrum.

   Figure 26: GUI of especget showing source and background spatial selection expressions.

   

Once all the spectral products are available, the SAS task specgroup allows further processing of the spectral files by performing a user-defined grouping of channels in the spectrum. The task also links the associated files by adding the information to the header of the source spectral file, useful for example when working with the software package Xspec[28]. The grouping of the spectrum can be done in several ways (see the description of the task for information on grouping options) included grouping based on minimum number of counts per channel, and several based on statistical criteria.

Note: especget writes the names of the created files into the source spectrum header keywords BACKFILE, RESPFILE, ANCRFILE. These may be automatically read by spectral fitting programmes to link the files and perform area weighted background subtraction. especget applies the following default event selections, for pn '(FLAG==0) && (PATTERN<=4)' and for MOS '#XMMEA_EM && (PATTERN<=12)'. especget also takes care of the different spectral ranges and binnings for pn and MOS spectra that need to be applied.