tokens: X-ray peak and a separation from continuum X-ray dominant region (DeLaney et al. 2004 ; Helder Vink 2008 ). On the other hand, we defined “south west,” “inner”, and “forward shock” regions as the “non-thermal dominant” region (light blue regions in Figure 2 ). Helder Vink ( 2008 ) and DeLaney et al. ( 2004 ) show the area dominated by a harder spectrum. As a matter of fact, these regions manifest themselves in hard X-rays. Our image also shows the inner region is bright with hard X-rays. In addition, the forward shock has a featureless non-thermal emission (e.g., Hughes et al. 2000 ; Bamba et al. 2005 ). In the forward shock, it was found that the average proper motion of Cassiopeia A is 030 yr −1 (Patnaude Fesen 2009 ), so we corrected the forward shock region of each year with this value (see the Appendix ). 3.2. Spectra With the regions defined in the Section 3.1 , we extracted spectra using a custom pipeline based on specextract in CIAO script. Here, the background spectra were extracted from outside of the whole SNR region defined in Figure 2 . Since X-ray emission from Cassiopeia A is very strong, the background contribution is almost negligible for a time variation estimation. The background fraction of the whole SNR is only ∼3% in the 4.2–7.3 keV band and this is almost constant value from 2002 to 2013. In 2000, this fraction shows a larger value (∼7%) presumably because of an increase in the charged particle flux experienced during this observation. After background subtraction, we fitted the 4.2–7.3 keV band spectra at each epoch with a power-law model and a Gaussian line with XSPEC version 12.8.2. The best-fit parameters are summarized in Table 2 . Table 2 Best-fit Parameters of the ACIS Spectra a Note. a The errors are at 1 σ confidence level. Figure 3 shows the spectra in the 4.2–7.3 keV band taken from the six regions. As described in Section 3.1 , the two bright non-thermal dominant regions south west and inner are remarkable in the hard X-ray continuum flux (Figures 1 and 3 ). Accordingly, the photon indexes of these regions are ∼2.6–3.0 while those of the thermal regions are 3.0–3.4. In the forward shock and whole SNR, photon indexes and evolutions are slightly different from the results in Patnaude et al. ( 2011 ). This is probably due to a difference in the energy band used in the spectral fitting. The fit residuals are larger for the thermal dominant regions than for the non-thermal regions. This is because weak thermal lines such as Cr–K (5.6 keV) appeared in the band. Figure 3. Time variation of the equivalent width of the Fe–K line (plot in the middle) and observed spectra in the 4.2–7.3 keV band (on both sides) at each region (whole SNR: WS, east: E, inner: I, north west: NW, south west: SW, forward shock: FS). In the central plot, the solid lines show best-fit linear models. Individual spectra were fitted with a model composed of a power law and a Gaussian. The error bars of all figures are 1 σ . As shown in Figure 3
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