  Before the launch of the Rossi X-ray Timing Explorer (RXTE) satellite, the
differences in the rapid X-ray variability between the two main types of
neutron star binaries (i.e., the Z and atoll sources) could be explained by
invoking different mass accretion rates and magnetic field strengths. However,
the results obtained with RXTE now show that these systems are more similar
than previously thought and although differences in mass accretion rate are
still likely, the differences in the magnetic field strength have become
questionable. The great similarities between the neutron star systems and the
black-hole candidates at low mass accretion rates also point towards a similar
origin of their timing phenomena indicating that the presence or absence of a
solid surface, a magnetic field, or an event horizon do not play a significant
role in the production mechanisms for the rapid X-ray variability.
