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Z sources are bright neutron-star X-ray binaries, accreting at around theEddington limit. We analyze the 68 RXTE observations (270 ks) of Sco-like Zsource GX 17+2 made between 1999 October 3-12, covering a complete Z track. Wecreate and fit color-resolved spectra with a model consisting of a thermalmulticolor disk, a single-temperature-blackbody boundary layer and a weakComptonized component. We find that, similar to what was observed for XTEJ1701-462 in its Sco-like Z phase, the branches of GX 17+2 can be explained bythree processes operating at a constant accretion rate Mdot into the disk:increase of Comptonization up the horizontal branch, transition from a standardthin disk to a slim disk up the normal branch, and temporary fast decrease ofthe inner disk radius up the flaring branch. We also model the Comptonizationin an empirically self-consistent way, with its seed photons tied to thethermal disk component and corrected for to recover the pre-Comptonized thermaldisk emission. This allows us to show a constant Mdot along the entire Z trackbased on the thermal disk component. We also measure the upper kHz QPOfrequency and find it to depend on the apparent inner disk radius R_in (priorto Compton scattering) approximately as frequency \propto R_in^(-3/2),supporting the idenfitication of it as the Keplerian frequency at R_in. Thehorizontal branch oscillation is probably related to the dynamics in the innerdisk as well, as both its frequency and R_in vary significantly on thehorizontal branch but become relatively constant on the normal branch.

The Spectral Evolution along the Z track of the Bright Neutron Star X-ray Binary GX 17+2