Reproducibility of Non—X-Ray Backgroundfor the X-Ray Imaging Spectrometer aboard Suzaku

摘要

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 highaccuracy is important to subtract them from the spectra of on-source observations. We are constructing an NXBdatabase by collecting XIS events when the the dark Earth covers the XIS FOV. The total exposure time of theNXB data is presently about 785 ks for each XIS. It is found that the count rate of the NXB anti-correlates withthe cut-off-rigidity and correlates with the count rate of the PIN upper discriminator (PIN-UD) in the Hard X-rayDetector aboard Suzaku. We thus model the NXB spectrum for a given on-source observation by employing eitherof these parameters, and obtain a better reproducibility of the NXB for the model with PIN-UD than that withthe cut-off-rigidity. The reproducibility of the NXB model with PIN-UD is 4.55-5.63% for each XIS NXB in the1-7 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 the1-7 keV band, and is almost comparable to that in the 5-12 keV band.