How to extract background spectra from blank fields event lists

Procedure

1. Set up your SAS environment (see Prerequisites for this thread at the top of the page).
    
2. Create an EPIC cleaned and filtered for particle background event file for your observation (see Prerequisites for this thread at the top of the page). Let the name of the filtered file be: EPICclean.fits.
    
3. Extract an image of the observation in detector coordinates:
   
     evselect table=EPICclean.fits imagebinning=binSize imageset=EPICimage.fits withimageset=yes \
       xcolumn=DETX ycolumn=DETY ximagebinsize=80 yimagebinsize=80
    
4. Display the image:
   
     imgdisplay withimagefile=true imagefile=EPICimage.fits
    
5. Identify the center coordinates and radius of the source spectrum extraction region. Let's assume that they are:
   
     DETX(center) = 100
     DETY(center) = -100
     Radius = 6000

    

6. Extract the source spectrum from the identified region:
   
   For MOS:
     evselect table=EPICclean.fits \
       withspectrumset=yes spectrumset=EPICsource_spectrum.fits \
       energycolumn=PI spectralbinsize=5 withspecranges=yes specchannelmin=0 specchannelmax=11999 \
       expression='((DETX,DETY) IN circle(100,-100,6000))'
   
   For pn:
     evselect table=EPICclean.fits \
       withspectrumset=yes spectrumset=EPICsource_spectrum.fits \
       energycolumn=PI spectralbinsize=5 withspecranges=yes specchannelmin=0 specchannelmax=20479 \
       expression='((DETX,DETY) IN circle(100,-100,6000))'
    
7. Let the name of the blank sky event list be (see Prerequisites for this thread at the top of the page on how to obtain one of these files): blank_sky_event_lists.ds
    
8. Add the keyword DATE-OBS to the primary and events extension of the blank sky event file, for example utilising the FITS file editor fv and adding the date value of the observation event file (see the watchout notes of the blank sky web pages).
   
   These keywords can also be added to the blank sky event file using FTOOLS commands, for example:
   
     fparkey '2004-07-12T12:30:00.0' blank_sky_event_lists.ds+0 DATE-OBS add=yes
     fparkey '2004-07-12T12:30:00.0' blank_sky_event_lists.ds+1 DATE-OBS add=yes
    
9. The blank sky event lists have been very conservatively filtered, to allow leverage for a user to further filter the data to the level that they desire. Therefore, it is strongly suggested that a user filters their blank sky event list to a level suitable for their observation and analysis using the same criteria applied for the generation of the source spectrum (see Prerequisites for this thread at the top of the page for filtering event files). Let the name of the filtered blank sky event file be: blank_sky_event_lists_clean.ds
    
10. Perform the same operation as for the source extraction region and extract the background spectrum from the same region in detector coordinates as the source, using the same event selection expression:
    
    For MOS:
      evselect table=blank_sky_event_lists_clean.ds \
        withspectrumset=yes spectrumset=background_spectrum.fits \
        energycolumn=PI spectralbinsize=5 withspecranges=yes specchannelmin=0 specchannelmax=11999 \
        expression='((DETX,DETY) IN circle(100,-100,6000))'
    
    For pn:
      evselect table=blank_sky_event_lists_clean.ds \
        withspectrumset=yes spectrumset=background_spectrum.fits \
        energycolumn=PI spectralbinsize=5 withspecranges=yes specchannelmin=0 specchannelmax=20479 \
        expression='((DETX,DETY) IN circle(100,-100,6000))'
     
11. From this point onwards, with the source and background spectral files, follow the thread for EPIC spectrum extraction to generate a redistribution matrix, an ancillary file and to calculate the area of source and background region (BACKSCALE) used to make the spectral files (see Prerequisites for this thread at the top of the page and the watchout notes of the blank sky background web pages). Please, note that the threads for EPIC spectrum extraction is focused on the analysis of point-like sources. In particular, the call to the SAS task arfgen to generate the ancillary file for extended sources, requires the use of the parameter extendedsource=yes. It is also advised that the call to arfgen includes an externally-created detector map which is more suitable for the analysis of extended sources (see for more details the documentation for arfgen).

It is also possible to extract a background spectrum from the blank sky event lists using a spectral extraction region defined in sky coordinates. In this case, one needs to recast the blank sky event list to mimic the nominal pointing direction of the observer's field, using, for instance, the SAS task attcalc:

  attcalc eventset=blank_sky_event_lists.ds attitudelabel=fixed withatthkset=N \
   refpointlabel=user nominalra=... nominaldec=...

where nominalra and nominaldec are the celestial coordinates of the central reference point which can be user defined by setting refpointlabl to user. For the case where the blank sky background field is matched to the source observation field, these values should be RA_NOM, DEC_NOM as they appear in the header of the source observation event file, which correspond to the boresight RA/Dec values of the prime instrument (i.e. the proposed pointing).