  We combined a principal component analysis (PCA) and spectroscopy to
investigate the origin of the soft excess in narrow-line Seyfert 1 galaxy Ark
564 with XMM-Newton observations over a period of ten years. We find that the
principal components in different epochs are very similar, suggesting stable
variability patterns in this source. More importantly, although its spectra
could be equally well fitted by the two soft excess models, simulations show
that the principal components from the relativistically smeared reflection
model match the data well. At the same time, the principal components from the
warm corona model show significant inconsistency. This finding indicates that
the soft excess in Ark 564 originates from the relativistically smeared
reflection, rather than the Comptonization in the warm corona, thereby favoring
the reflection origin or the "hybrid" origin of the soft excess. Furthermore,
the presence of the narrow absorption features in the spectra suggests that the
soft excess is unlikely to originate from absorptions due to possible
outflowing winds. Our results indicate that the PCA coupled with spectral
analysis is a promising approach to exploring the origin of the soft excess in
active galactic nuclei (AGNs).
