  We have used INTEGRAL & RXTE data to investigate the timing properties of the
source in correlation with its spectral states as defined by different
positions in the colour-colour diagram. The source shows two distinct branches
in the colour-colour diagram that resemble those of the Z sources. The hard
branch (similar to the horizontal branch of Z sources) is characterised by a
low-amplitude change of the hard colour compared to the change in the soft
colour. In the soft branch (analogue to the normal branch) the amplitude of
variability of the hard colour is about three times larger than that of the
soft colour. As the count rate decreases the source moves up gradually through
the soft to the hard branch. The aperiodic variability (excluding the pulse
noise) consists of band-limited noise (represented by three broad Lorentzian
components) and two QPOs at 0.05 Hz and 0.22 Hz. The strength of the lower
frequency QPO increases as the source approaches the hard branch (similar to
HBOs in Z sources). The higher frequency QPO reaches maximum significance when
the source is in the middle of the branch (like NBOs). We have performed the
first measurements of phase lags in the band limited noise below 8 Hz in an
accreting X-ray pulsar and found that soft lags dominate at high frequencies.
Above the pulse frequency (0.23 Hz), the amplitude of the lag increases as the
X-ray flux increases. The Z topology appears to be a signature of the neutron
star binaries as it is present in all types of neutron-star binaries (Z, atoll
and, as we show here, in accreting pulsars as well). However, the motion along
this track, the time scales through the different branches of the diagram and
the aperiodic variability associated with portions of the Z track differ for
each subclass of neutron-star binary.
