Links Between Z Sources and Atoll Sources
  It is known that the Z and atoll sources are two typical types of neutron-star sources in low mass X-ray binaries (LMXBs), which present very different Q-$\nu$ relations of lower kHz QPOs. We propose that the Z and atoll sources are two different phases in the evolutionary track of neutron star in LMXBs, instead of two types of distinct sources. 
J. Wang, H.-K. Chang
astro-ph.HE