Accreting neutron stars provide unique environments where the accreting matter
experiences physical processes under strong gravity and magnetic elds. The X-ray
emission from an accreting neutron star reflects the effects of such unique physical
processes. The X-ray emission from accreting neutron stars exhibits time variability due
to anisotropic emission caused by three-dimensional structures of the accretion flow and
the neutron star, and the reprocess by the circumstellar medium. However, the relation
between observational time variabilities and these physical conditions has not been
deeply investigated since the time variability emerges as a consequence of a complicated
mixture of these effects.

We studied the X-ray spectral variabilities of two luminous accreting neutron stars, Cen
X-3 and Her X-1, with NuSTAR observatory. We analyzed the spectral variabilities along
both orbital and spin phases, which can be linked to three-dimensional structures of the
circumstellar medium and accretion stream, respectively. Our spectral analysis revealed
that the orbital-phase variability can be explained by the obscuration caused by the
inhomogeneous stellar wind. The accretion rate of these two sources is stable, and the
variant spectral shape is caused by different obscuration rates between the neutron star
and the accretion disk. We also studied the spin-phase variabilities of these sources by
comparing the observation with our Monte-Carlo simulation. Our simulation calculates the
physical processes under strong magnetic fields and derives the three-dimensional
emission spectrum from the accretion column. It successfully reproduced the energy
dependence of the pulse profile shape. The comparison of the observation and the
simulation revealed that the emission from the accretion column is mainly composed of
the direct fan beam and the reflection component from the neutron star surface. In this
seminar, we first review our spectral analysis and then discuss possible physical
pictures derived from our analysis.