Rossi X-ray Timing Explorer observations of the first transient Z source XTE J1701-462:: Shedding new light on mass accretion in luminous neutron star X-ray binaries

We report on the first 10 weeks of RXTE observations of the X-ray transient XTE J1701 - 462 and conclude that it had all the characteristics of the neutron star Z sources, i.e., the brightest persistent neutron star low-mass X-ray binaries. These include the typical Z-shaped tracks traced out in X-ray color diagrams and the variability components detected in the power spectra, such as kHz QPOs and normal and horizontal branch oscillations. XTE J1701 - 462 is the first transient Z source and provides unique insights into mass accretion rate ((m) over dot) and luminosity dependencies in neutron star X-ray binaries. As its overall luminosity decreased, we observed a switch between two types of Z source behavior, with the branches of the Z track changing their shape and/or orientation. We interpret this as an extreme case of the more moderate long-term changes seen in the persistent Z sources and suggest that they result from changes in. m. We also suggest that the Cyg-like Z sources (Cyg X- 2, GX 5 - 1, and GX 340+0) are substantially more luminous (> 50%) than the Sco-like Z sources (Sco X- 1, GX 17+2, and GX 349+2). Adopting a possible explanation for the behavior of kHz QPOs, which involves a prompt as well as a filtered response to changes in. m, we further propose that changes in. m can explain both movement along the Z track and changes in the shape of the Z track. We discuss some consequences of this and consider the possibility that the branches of the Z will smoothly evolve into the branches observed in X-ray color diagrams of the less luminous atoll sources, although not in a way that was previously suggested.