If requested (runbackground=Y), the task creates a background lightcurve, as well as a background spectrum 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 which case the total lightcurve is produced. The usage of background masks can be controlled by parameter withmask (default value: yes). The background masks are created by task badpixfind using threshold parameters, appropriate for the detection of real X-ray sources. The background lightcurves and corresponding masks are created in the 7.0 - 15.0 keV energy band, while masks created in the 0.2-10.0 keV energy band are used for the creation of the background spectrum.
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<500'or using the count rate column
cd .. tabgtigen table=rate_bkg.fits gtiset=bkg_GTI.fits expression='RATE<8'where the output rate is in counts ks arcmin. Typical low-background rates for PN are of the order 3 - 5 counts ks arcmin in the 7.0 to 15.0 keV energy range.
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.