  We studied the aperiodic X-ray timing behaviour of the accreting millisecond
pulsar SAX J1808.4-3658. The source was recently found to be the first
accreting millisecond pulsar that shows the kilohertz quasi-periodic
oscillations (kilohertz QPOs) that are found in many other X-ray binaries with
accreting neutron stars. The high frequency of these signals reflects the short
dynamical time scales in the region near the compact object where they
originate. We find that in addition to the kilohertz QPOs SAX J1808.4-3658
shows several low frequency timing features, based on which the source can be
classified as a so-called atoll source. The frequencies of the variability
components of the atoll sources follow a universal scheme of correlations. The
correlations in SAX J1808.4-3658 are similar but show a shift in upper
kilohertz QPO frequency. This discrepancy is perhaps related to a stronger or
differently configured magnetic field.

Astrophysics