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Дата изменения: Fri Jan 10 23:58:50 2003
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XMM-Newton Science Analysis System


epchain (epchain-8.30.5) [xmmsas_20030110_1802-5.4.1]

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Screening of periods of high background

If requested (runbackground=Y), the task also creates a background lightcurve for the imaging modes (FF, eFF, LW, SW) using background masks (these are not used for the fast modes as there is no useful background region, in this case the total lightcurve is produced). Depending on the individual observation and on the requested timebinsize one can then create a GTI file via tabgtigen (e.g.):

  tabgtigen table=rate_bkg.fits gtiset=bkg_GTI.fits expression='COUNTS<200'

or using the count rate column

  tabgtigen table=rate_bkg.fits gtiset=bkg_GTI.fits expression='RATE<60'

where the output rate is in counts ks$^{-1}$ arcmin$^{-2}$. Typical low-background rates for PN are of the order of 10 counts s$^{-1}$ in the $0.2-8$keV range for the whole field-of-view (without bright sources, i.e. counts per time bin $\sim 10\times{\tt timebinsize}$. If you decide to use the hard energy band ($7.0-15.0$keV) instead of the soft one ($0.16-10.0$keV) via the switch withhardmask=Y then a recommended threshold is

  tabgtigen table=rate_bkg.fits gtiset=bkg_GTI.fits expression='RATE<6'

The GTI file can then be used to screen the event list via evselect (e.g.)

  evselect expression='GTI(bkg_GTI.fits,TIME)' ...

As the GTIs strongly depend on the intended science this screening is not performed and only background masks, the lightcurve, and a GTI file are created by epchain.