  X-ray reverberation lags from the vicinity of supermassive black holes have
been detected in almost 30 AGN. The soft lag, which is the time delay between
the hard and soft X-ray light curves, is usually interpreted as the time
difference between the direct and reflected emission, but is alternatively
suggested to arise from the direct and scattering emission from distant clouds.
By analysing the archival Suzaku observations totalling an exposure time of ~
770 ks, we discover a soft lag of $10\pm3.4$ ks at $9.58\times10^{-6}$ Hz in
the luminous quasar PDS 456, which is the longest soft lag and lowest Fourier
frequency reported to date. In this study, we use the maximum likelihood method
to deal with non-continuous nature of the Suzaku light curves. The result
follows the mass-scaling relation for soft lags, which further supports that
soft lags originate from the innermost areas of AGN and hence are best
interpreted by the reflection scenario. Spectral analysis has been performed in
this work and we find no evidence of clumpy partial-covering absorbers. The
spectrum can be explained by a self-consistent relativistic reflection model
with warm absorbers, and spectral variations over epochs can be accounted for
by the change of the continuum, and of column density and ionization states of
the warm absorbers.
