Active Galactic Nuclei (AGN) and black-hole binaries (BHB) often exhibit X-ray broad
iron spectral features, which may be explained either by the ¡Èdisk-line¡É model or the
¡Èpartial covering¡É model. It is hardly possible to determine which models are valid
from time-averaged spectral analysis, thus X-ray spectral variabilities have been
investigated to constrain spectral models. To that end, it is crucial to study iron
structure of BHBs in detail at short time-scales. This is made possible with the Suzaku
XIS Parallel-sum clocking (P-sum) mode, which has the CCD energy-resolution as well as a
time-resolution of 7.8 ms. We have carried out systematic calibration of the P-sum mode,
and investigated spectral variabilities of GRS 1915+105 at sub-seconds for the first
time. Consequently, we found that the spectral variabilities of GRS 1915+105 does not
show iron features at sub-seconds. This is totally different from AGN variabilities,
where the variability significantly drops at the iron energy band. This indicates that
spectral variability timescales of AGN and BHB are not simply normalized by black-hole
masses, which is naturally understood in the framework of the ¡Èpartial covering¡É
model.