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OI Fluorescent Line Contamination in Soft X-Ray Diffuse Background Obtained with Suzaku/XIS

The quantitative measurement of OVII line intensity is a powerful method for understanding the soft X-ray diffuse background. By systematically analyzing the OVII line intensity in 145 high-latitude Suzaku/XIS observations, the flux of OI fluorescent line in the XIS spectrum, contaminating the OVII line, is found to have an increasing trend with time especially after 2011. For these observations, the OVII line intensity would be overestimated unless taking into consideration the OI fluorescent line contamination. Since the OI line emission originates from solar X-rays, this increase suggests that incident solar X-rays at the OI fluorescence energy tend to be larger than the early phase of Suzaku observations (2005 - 2010).