MOS blank field background event lists

EPIC background spectra are normally extracted from regions of the observation field-of-view, once "contamination" due to individual discrete celestial sources has been removed. This technique is fully appropriate when analyzing spectra of point-like sources in full window modes. However, there are cases where the determination of the background from the same observation is difficult, either due to the intrinsic extended nature of a celestial object a user is interested to analyze, or because a bright source was observed in a windows mode. In the latter case, it is sometimes impossible to identify field-of-view regions, where the target source does not substantially contribute to the total X-ray counts.

In order to cope with this situation, blank sky event lists can be downloaded from the XMM-Newton Science Operations Center web site. This thread briefly explain how to use them.

This thread does not deal with the rather complex problem of a careful determination of the background normalization appropriate for a given observation. Users are referred to the EPIC background analysis web pages for more information.

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

extract an image of the observation in detector coordinates

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

display the image

imgdisplay withimagefile=true imagefile=MOSimage.fits

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 = 8 x 80 = 640

extract the source spectrum from the identified region:

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) && ((DETX,DETY) IN circle(100,-100,640))'

download the appropriate blank field event list from the blank sky event lists repository (see here for an explanation of the naming convention). Let it be: MOSbackground.ds.

extract the background spectrum from the same region in detector coordinates as the source, using the same event selection expression:

evselect table=MOSbackground.ds withspectrumset=yes spectrumset=MOSbackground_spectrum.fits \
  energycolumn=PI spectralbinsize=15 withspecranges=yes specchannelmin=0 specchannelmax=11999 \
  expression='#XMMEA_EM && (PATTERN<=12) && ((DETX,DETY) IN circle(100,100,640))'

back to the MOS spectrum extraction thread, step n.13

It is in principle possible as well to extract background spectra from blank field event lists, if the spectrum accumulation regions are defined in sky coordinates. In this case, one needs to recast the blank field events to mimic the nominal pointing direction of the observer's field, using, for instance, the SAS task attcalc:

attcalc eventset=MOSbackground.ds attitudelabel=fixed withatthkset=N refpointlabl=user nominalra=... nominaldec=...