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Abstract
One of the advantages of the X-ray Imaging Spectrometer (XIS) system on board Suzaku
is its low and stable non-X-ray background (NXB). In order to make the best use of
this advantage, modeling the NXB spectra with high accuracy is important to subtract
them from the spectra of on-source observations. We construct an NXB database by collecting
XIS events when the dark Earth covers the XIS FOV. The total exposure time of the
NXB data is about 785 ks for each XIS. It is found that the count rate of the NXB
anti-correlates with the cut-off-rigidity and correlates with the count rate of the
PIN upper discriminator (PIN-UD) in Hard X-ray Detector on board Suzaku. We thus model
the NXB spectrum for a given on-source observation by employing either of these parameters
and obtain a better reproducibility of the NXB for the model with PIN-UD than that
with the cut-off-rigidity. The reproducibility of the NXB model with PIN-UD is 4.55-5.63%
for each XIS NXB in the 1-7 keV band and 2.79-4.36% for each XIS NXB in the 5-12 keV
band for each 5 ks exposure of the NXB data. This NXB reproducibility is much smaller
than the spatial fluctuation of the cosmic X-ray background in the 1-7 keV band, and
is almost comparable to that in the 5-12 keV band.