Rxte 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 ten weeks of Rossi X-ray Timing Explorer observations of the X-ray transient XTE J1701–462. Large count rate variations were accompanied by changes in the source’s spectral and variability properties. Based on comparisons with other sources we conclude that XTE J1701–462 had all the characteristics of the neutron star Z sources, i.e., the class of the brightest persistent neutron star low-mass X-ray binaries. These include the typical Z-shaped tracks traced out in X-ray color diagrams on time scales of hours and the variability components detected in the power spectra, such as kHz quasi-periodic oscillations (QPOs) and normal and horizontal branch oscillations. XTE J1701–462 is the first transient Z source and provides unique insights into mass accretion rate (ṁ) and luminosity dependencies in neutron star X-ray binaries. As its overall luminosity decreased, we observed that XTE J1701–462 switched between two types of Z-source behavior never previously identified as clearly in the same Z source, with most of the branches of the Z-track changing their shape and/or orientation. We interpret this switch as an extreme case of the more moderate longterm changes seen in the persistent Z sources and suggest that they are the result of changes in ṁ. We also suggest that the Cyg-like Z sources (Cyg X-2, GX 5-1, and GX 340+0) might be 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 ṁ, we further propose that changes in ṁ can explain both movement along the Z track and changes in the shape of the Z track itself. We discuss some consequences of this and consider the possibility that the branches of the Z will smoothly evolve into the branches observed in the X-ray color diagrams of the less luminous atoll sources, although not in a way that was previously suggested. Subject headings: accretion, accretion disks — stars: individual (XTE J1701–462) — stars: neutron — X-rays: stars