Reproducibility of Non-X-Ray Background for the X-Ray Imaging Spectrometer aboard Suzaku

One of the advantages of the X-ray Imaging Spectrometer (XIS) system aboard 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 are constructing an NXB database by collecting XIS events when the the dark Earth covers the XIS FOV. The total exposure time of the NXB data is presently about 785ks 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 the Hard X-ray Detector aboard 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 1-7keV band and 2.79-4.36% for each XIS NXB in the 5-12keV band for each 5ks exposure of the NXB data. This NXB reproducibility is much smaller than the spatial fluctuation of the cosmic X-ray background in the 1-7keV band, and is almost comparable to that in the 5-12keV band.