XMM-Newton Science Analysis System: User Guide

4.8.1 Generating spectra

EPIC calibrated event lists must be filtered to generate spectra. As for images, this filtering is normally performed in two steps. First, the event lists are screened to reject spurious data and to select only events which contain information of sufficient quality for further scientific analysis. Secondly, the screened data are filtered to construct data subsets adapted to specific spectral analysis.

For the latest information on general recommendations for a conservative spectral analysis (i.e. recommendations about where the data should be taken from the CCD, which energy and pattern range should be used, which event quality flags should be selected), the user is strongly advised to check the related sections in document "EPIC status of calibration and data analysis" (XMM-SOC-CAL-TN-0018, [11]), available on the XMM-Newton SOC Calibration Web page at http://www.cosmos.esa.int/web/xmm-newton/calibration.

See also the SAS threads at:

http://www.cosmos.esa.int/web/xmm-newton/sas-threads

The screening and filtering activities can be performed in an interactive manner with the SAS task xmmselect. Basically, there are two possible approaches offered by xmmselect to generate EPIC spectral products:

• generate source and background spectra separately via the xmmselect product selection "OGIP Spectrum". This approach requires more analysis steps but is described here as users might be interested in details,
• generate source and background spectra (together with related response files) with the meta-task especget called via the xmmselect product selection "OGIP Spectral Products".

For both cases, an EPIC image must be created and displayed with xmmselect running on a filtered (or the original) event list (see § 4.7.2). Note that it is not enough to display an image through ds9 for xmmselect to be able to communicate with it, the ds9 window must have been launched by xmmselect.

In the following, the analysis steps for the "OGIP Spectrum" approach are described:

1. In the ds9 window, create a region for the source of interest. Click once on the ds9 image and a region circle will appear (other region shapes are available as well). 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, click the "2D region" button in the xmmselect GUI. This transfers the region information into the "Selection expression" text area in xmmselect. A newly defined ds9 region file can optionally be saved to disk via the ds9 "Region" $\rightarrow$ "Save Regions..." menu and re-loaded for further analysis steps via the "Region" $\rightarrow$ "Load Regions..." option.

   Note: For pn data of bright sources and of sources with narrow lines it might be good to extract two spectra and corresponding backgrounds, response and ancillary files: one set for single pixel events (PATTERN==0) and another set for doubles (PATTERN IN [1:4]). Fitting these two spectra simultaneously will show if there are any problems with pile-up (see § 4.5) and - as the energy calibration for singles is slightly better than the one for doubles - will show the line features at highest energy resolution in the single events spectra.

2. To extract the spectrum, first click the circular button next to the PI column in the xmmselect GUI. Next click the "OGIP Spectrum" button. Select the "Spectrum" page of the evselect GUI to set the file name and binning parameters for the spectrum. For example, set spectrumset to src_spectrum.fits. The parameter spectralbinsize (the size of each spectral bin in instrumental eV) shall be set to the same value (e.g., 5), if the user wishes to merge the EPIC -MOS and -pn spectra into a single EPIC spectrum (see the "Combining the spectra of the 3 EPIC cameras" thread;
   http://www.cosmos.esa.int/web/xmm-newton/sas-threads).
   Otherwise, the user is free to choose the bin size independently for each camera. withspecranges must be checked, specchannelmin set to 0, and specchannelmax set to 11999 for the MOS or 20479 for the pn. Figure 21 shows a plot of an example output spectrum which is automatically displayed in a grace window upon generation by xmmselect.

   Figure 21: Example of count spectrum of a source which is automatically displayed in a grace window upon generation by xmmselect.

   

   Note: xmmselect performs the calculation of the BACKSCAL factor, which takes into account CCD gaps, bad pixels and the size of the extraction region, on the fly during the spectrum generation. If evselect is used instead for the extraction of the source and background spectra, the BACKSCAL factor must be calculated explicitly by executing the backscale task before any subsequent quantitative analysis, e.g. with Xspec, can be performed.

3. For checking whether pileup (§ 4.5) might be a problem, create a source event file by checking the keepfilteroutput and withfilteredset boxes on the evselect "General" page and provide a filteredset name, e.g. src_evlist.fits, for the resultant file. For this event file, remove the &&(PATTERN<=4) phrase for the pn so that single, double, triple, and quadruple events are all included. This filtered event list should be used as input file for the pattern analysis to be performed by epatplot (see § 4.5).
4. To extract a background spectrum from a source-free area, first remove the spatial selection (previously defined for the source) from the "Selection expression" window in xmmselect. Next repeat step 1) except using now the "Region" option of ds9 to define the background area and then click the "2D region" button. This will transfer the background region description to the "Selection expression" in xmmselect. Finally, repeat step 2) except setting now the spectrumset parameter to a different file name, e.g. bkg_spectrum.fits.

   Note: Background extraction from a source-free area might be a problem in case of crowded fields. As a general advice for MOS, the source-free background region should be extracted at roughly the same off-axis angle as the position of the source. For pn, the recommendation is to select a source-free background region at the same RAWY position on the chip as the source (RAWY being the long axis of the CCD). So if the pn source is located e.g. at RAWY line 150 on CCD 4, one should aim for selecting the background from around line 150 on the same chip or, at least, from a chip belonging to the same quadrant.

In the case of extended sources, where virtually no emission free regions exist on the CCD, the user is advised to make use of EPIC background files and tools available through the SOC pages (further details are given in § 4.6). An alternative approach for MOS might be to extract source-free background regions from the outer CCDs (which always are collecting data in imaging mode).

In the case of EPIC timing and pn burst mode observations that generally are performed for bright point-like sources, the background will usually not be an issue. Note that in these modes, the RAWY coordinate is not giving spatial but timing information and the source is visible as a bright strip when plotting RAWX against RAWY. In the case of the MOS cameras, the timing strip is only 100 pixels wide and if a background spectrum needs to be extracted it should be taken from the outer CCDs which are collecting data in imaging mode. In the case of the pn, background regions can be extracted in emission-free strips parallel to the readout direction (i.e. defining a spatial filter expression in RAWX only - a selection in RAWY would incorrectly exclude certain time intervals) excluding the region with registered events from the source.

Details on how to extract source and background regions for the case of EPIC timing mode are given at:

http://www.cosmos.esa.int/web/xmm-newton/sas-threads

for the case of pn and MOS.

For the analysis of pn burst mode data special care has to be taken in the way of selecting the source and background regions. RAWY may have to be chopped in order to exclude contamination from the source, see e.g. the analysis of burst mode data of the Crab in Kirsch et al. (2006) [10], section 3, available online as XMM-SOC-CAL-TN-0069 [10].

Some information on the analysis of extended objects, particularly where a determination of the background is difficult from the individual data set, is provided in § 4.6.