THE SPECTRAL EVOLUTION ALONG THE Z TRACK OF THE BRIGHT NEUTRON STAR X-RAY BINARY GX 17+2

Abstract

Z sources are bright neutron star X-ray binaries, accreting at around the Eddington limit. We analyze the 68 RXTE observations (~270 ks) of Sco-like Z source GX 17+2 made between 1999 October 3 and 12, covering a complete Z track. We create and fit color-resolved spectra with a model consisting of a thermal multicolor disk, a single-temperature-blackbody boundary layer and a weak Comptonized component. We find that, similar to what was observed for XTE J1701-462 in its Sco-like Z phase, the branches of GX 17+2 can be explained by three processes operating at a constant accretion rate [dot over M] into the disk: increase of Comptonization up the horizontal branch (HB), transition from a standard thin disk to a slim disk up the normal branch (NB), and temporary fast decrease of the inner disk radius up the flaring branch. We also model the Comptonization in an empirically self-consistent way, with its seed photons tied to the thermal disk component and corrected for to recover the pre-Comptonized thermal disk emission. This allows us to show a constant [dot over M] along the entire Z track based on the thermal disk component. We also measure the upper kHz quasi-periodic oscillation frequency and find it to depend on the apparent inner disk radius R [subscript in] (prior to Compton scattering) approximately as frequency α R[superscript –3 over 2] [subscript in], supporting the identification of it as the Keplerian frequency at R [subscript in]. The HB oscillation is probably related to the dynamics in the inner disk as well, as both its frequency and R [subscript in] vary significantly on the HB but become relatively constant on the NB.