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) && ((BETA_CORR,XDSP_CORR) IN \ REGION(P0136540101R2S002SRCLI_0000.FIT:RGS2_BACKGROUND))
What constitutes unacceptably high background is often a matter of personal judgement
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
GTI mechanism to flag periods of low background.
tabgtigen table=RGS2.background.FITS gtiset=RGS2.background.GTI.FITS \ expression='(RATE<1)'