and ancillary response files (ARF), respectively. The source mode of xissimarfgen was set to SKYFITS, which is appropriate for analyzing extended objects, and the XIS image file was input. Data from the hard X-ray detector, HXD, are not treated here because no signal was detected from NGC 4636; an upper limit of the hard X-ray emission is $9\times10^{-12}$ erg cm$^{-2}$ s$^{-1}$ (10–50 keV) for the 3% background uncertainty (Fukazawa et al. 2009).

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Spectral Analysis and Results
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Estimation of Background
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In order to study the emission from NGC 4636 correctly, we have to estimate the background accurately. This consists of the instrumental non X-ray background (NXB), the cosmic X-ray background (CXB) and the foreground Galactic X-ray emission (GXE).

First, we estimated the NXB spectra in each analysis region using the tool “xisntebgdgen”, which adds COR-sorted ($<$ 4GV, 4–13 GV in 1 GV step, and $>$ 13 GV) night-Earth spectra weighted by the exposure time for each COR in the NGC 4636 observation. Second, using the sky-averaged CXB parameters from HEAO-1 (Boldt et al. 1987) summarized in table 2, we estimated the CXB contribution by using an uniform-sky ARF, which is generated with “xissimarfgen” and 