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

TL;DR

This study analyzes RXTE observations of the neutron star X-ray binary GX 17+2, revealing how spectral states and QPO frequencies relate to accretion disk processes along the Z track, with implications for disk geometry and accretion physics.

Contribution

The paper introduces a comprehensive spectral model linking accretion disk states to spectral branches and QPOs in GX 17+2, demonstrating a constant accretion rate along the Z track and the relation between QPO frequency and inner disk radius.

Findings

Spectral states explained by three processes at constant accretion rate.

Inner disk radius correlates with kHz QPO frequency as R_in^(-3/2).

Inner disk dynamics influence horizontal branch oscillations.

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 3-12, 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 Mdot into the disk: increase of Comptonization up the horizontal branch, transition from a standard thin disk to a slim disk up the normal branch, 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 Mdot along the entire Z track based on the thermal disk component. We also measure the upper kHz QPO frequency and find it to depend on the apparent inner disk radius R_in (prior to Compton scattering) approximately as frequency \propto R_in^(-3/2), supporting the idenfitication of it as the Keplerian frequency at R_in. The horizontal branch oscillation is probably related to the dynamics in the inner disk as well, as both its frequency and R_in vary significantly on the horizontal branch but become relatively constant on the normal branch.