Spectral analysis of MOS point-like sources

All the analysis steps are performed with single SAS tasks started from the command line to explain the general method of generating spectral products and to show explicitly the usage and setting of task parameters. The users should note that the SAS metatask xmmselect allows them to interactively define source and background regions (via ds9) and to run backscale on the fly. Especially the xmmselect:Spectral Products generation method, which in addition offers an optimization of the source extraction region and which creates source and background spectra as well as related ancillary and redistribution files in one go, might in some cases be a GUI based alternative to the command line method described below. For more details on how to use xmmselect for the generation of EPIC spectra, the reader is referred to the Users' Guide to the XMM-Newton Science Analysis System.

1. set up your SAS environment (following the SAS start-up thread)

2. extract a single event (i.e. pattern zero only), high energy (E > 10 keV) light curve, to identify intervals of flaring particle background

evselect table=MOS.fits withrateset=Y rateset=rateMOS.fits \
  maketimecolumn=Y timebinsize=100 makeratecolumn=Y \
  expression='#XMMEA_EM && (PI>10000) && (PATTERN==0)'

dsplot table=rateMOS.fits x=TIME y=RATE

3. determine a threshold on the light curve counts, defining "low background" intervals (in our example: 0.35 cts/s), create a corresponding GTI file and use it to filter the event list:

tabgtigen table=rateMOS.fits expression='RATE<=0.35' gtiset=MOSgti.fits

evselect table=MOS.fits withfilteredset=Y filteredset=MOSclean.fits \
  destruct=Y keepfilteroutput=T \
  expression='#XMMEA_EM && gti(MOSgti.fits,TIME) && (PI>150)'

4. extract an image (sky coordinates in this example; extraction in detector - DET[XY] - coordinates is possible as well, and may be preferable for some specific scientific needs)

NOTE: arfgen/rmfgen do not support spectra extracted from a region defined in RAW coordinates

evselect table=MOSclean.fits imagebinning=binSize imageset=MOSimage.fits withimageset=yes \
  xcolumn=X ycolumn=Y ximagebinsize=80 yimagebinsize=80

5. display the image

imgdisplay withimagefile=true imagefile=MOSimage.fits

This command is equivalent to the following:

ds9 MOSimage.fits

6. select the region, from which the spectrum shall be accumulated, using the Region/Circle in ds9 (see Fig.1)




Fig.1: ds9 main window. A circular region (green circle) has been defined using the highlighted menu.

7. double-click with the cursor on the defined region. A window pops up, showing the properties of the region (Fig.2) (you need to set 'Coord -> Physical' and 'Radius -> Physical' to switch to physical coordinates). Write down the coordinates of the Center (30360.5,28400.5) and of the Radius (640) as they will be needed in step 10 to define the spatial filter expression.

(These values would also be propagated into a Selection Expression if pressing the "2D region" button in xmmselect...)




Fig.2: selection region properties window, pop'd-up by double-clicking on the region in the main ds9 window

8. If you want to see the center position in RA, Dec (J2000) coordinates, switch 'Coord -> WCS' (World Coordinate System) and select 'Equatorial J2000'. To diplay the radius of the selection region in arcsec, switch 'Radius -> WCS' and select 'ArcSec'. Units of skycoordinates (X,Y) are 0.05 arcsec, hence the radius in our example is 32 arcsec.

9. extract a source spectrum, using all the selection expressions defined so far & restricting the patterns to single and doubles

evselect table=MOSclean.fits withspectrumset=yes spectrumset=MOSsource_spectrum.fits \
  energycolumn=PI spectralbinsize=15 withspecranges=yes specchannelmin=0 specchannelmax=11999 \
  expression='#XMMEA_EM && (PATTERN<=12) && ((X,Y) IN circle(30360.5,28400.5,640))'

10. extract a background spectrum. Have a look at the "EPIC status of calibration and data analysis" document (XMM-SOC-CAL-TN-0018) for latest recommendations on how to select source and background regions. In the following, we assume that the background is extracted from a source-free region on the same CCD and at roughly the same off-axis angle as the source under investigation.

    evselect table=MOSclean.fits withspectrumset=yes spectrumset=MOSbackground_spectrum.fits \
      energycolumn=PI spectralbinsize=15 withspecranges=yes specchannelmin=0 specchannelmax=11999 \
      expression='#XMMEA_EM && (PATTERN<=12) && ((X,Y) IN circle(30720.5,26360.5,640))'

    If you are interested in learning how to extract the background spectra from blank sky event lists, please click here.

11. calculate the area of source and background region used to make the spectral files. The area is written into the header of the SPECTRUM table of the file as keyword BACKSCAL (if the spectrum is created via xmmselect, backscale will run automatically)

    backscale spectrumset=MOSsource_spectrum.fits badpixlocation=MOSclean.fits
    backscale spectrumset=MOSbackground_spectrum.fits badpixlocation=MOSclean.fits

12. generate a redistribution matrix

Currently there are two possible approaches:

a) use the SAS task rmfgen to create a redistribution matrix for your previsouly extracted spectrum:

rmfgen spectrumset=MOSsource_spectrum.fits rmfset=MOS.rmf

NOTE: This can take long (>30 min) on low-performance computers...

b) use the ready-made (canned) matrices available at the following URL: http://xmm.esac.esa.int/external/xmm_sw_cal/calib/epic_files.shtml

13. generate an ancillary file (for extended sources use extendedsource=yes detmaptype=flat or dataset)

NOTE: arfgen reads in the pattern range from the DSS information in the spectrum dataset, and accumulates the quantum efficiency curves over those patterns, which is then combined to the other consituents of the ARF. Be aware that the entire range of allowed patterns (0-31 for the MOS) are assumed if no pattern range is found in the DSS.

arfgen spectrumset=MOSsource_spectrum.fits arfset=MOS.arf withrmfset=yes rmfset=MOS.rmf \
  badpixlocation=MOSclean.fits detmaptype=psf

14. prepare the spectrum and link associated files

FTOOL: grppha: PHA filename: MOSsource_spectrum.fits

               output filename: MOSsource_spectrum.grp

               chkey BACKFILE MOSbackground_spectrum.fits

               chkey RESPFILE MOS.rmf

               chkey ANCRFILE MOS.arf

               group min 25 ! as an example

               exit

15. fit the spectrum

FTOOL: xspec: data MOSsource_spectrum.grp

16. NOTE ON PATTERN SELECTION IN MOS SPECTRA:

In general the user should use PATTERN 0-12. However, PATTERN 0 events can be used to minimise the effects of pile-up in bright sources and for sources in which the best-possible spectral resolution is crucial.
In case of MOS Timing mode observations PATTERN 0 only should be selected for the source and PATTERN 0,1,3 for the background spectra extracted in outer CCDs. For details and latest recommendations, see again XMM-SOC-CAL-TN-0018.