XMM-Newton Science Analysis System: User Guide


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5.7.2 Generating raw RGS light curves to assess the background

In addition to the corrected source light curves available using the task rgslccorr described below in § 5.17, xmmselect may be used to investigate other aspects of the time variability of RGS data using the [OGIP Rate Curve] button with suitable selection criteria.

Perhaps the principal practical use is for assessment of the background that often accounts for the majority of detected events over the whole instrument and which must therefore be explicitly quantified. The background is made up of several contributions and is usually weak enough to cause few problems: RGS spectra are photon limited more than 80% of the time. However, solar flares and other particle events can cause significant or in rare cases even overwhelming contamination. Rapid variability is also common so that only part of an observation may be affected so that it is possible to generate supplementary GTIs in order to exclude periods of unacceptably high background. Figure 37 shows the background light curve generated using:

(CCDNR = 9) && ((MLAMBDA,XDSP_CORR) IN \
  REGION(P0136540101R2S002SRCLI_0000.FIT:RGS2_BACKGROUND))

What constitutes unacceptably high background is often a matter of personal judgment depending on the type of analysis undertaken. High-contrast features like strong emission lines can often tolerate higher background levels than smooth continuum spectra and the overall source brightness is clearly a consideration. It is best to experiment. As a rule of thumb, data with CCD9 background rates above 1 count/s might be considered suspect and be excluded by using the GTI mechanism to flag periods of low background.

 tabgtigen table=RGS2.background.FITS gtiset=RGS2.background.GTI.FITS \
            expression='(RATE<1)'

Figure 37: RGS2 light curve of the background near Mkn421 generated with xmmselect. Although there are a few small flares, none are bad enough to consider removing from the source data.
\scalebox{0.6}{\includegraphics{RGS/Mkn421.RGS2.background.ps}}


next up previous contents
Next: 5.8 Rerunning the RGS processor rgsproc Up: 5.7 The use of xmmselect on RGS data Previous: 5.7.1 Generating RGS images
European Space Agency - XMM-Newton Science Operations Centre