THE SPECTRAL EVOLUTION ALONG THE Z TRACK OF THE BRIGHT NEUTRON STAR X-RAY BINARY GX 17+2
Author(s)Lin, Dacheng; Homan, Jeroen; Barret, Didier; Remillard, Ronald A
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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.
DepartmentMIT Kavli Institute for Astrophysics and Space Research
Lin, Dacheng, Ronald A. Remillard, Jeroen Homan, and Didier Barret. “THE SPECTRAL EVOLUTION ALONG THE Z TRACK OF THE BRIGHT NEUTRON STAR X-RAY BINARY GX 17+2.” The Astrophysical Journal 756, no. 1 (August 10, 2012): 34. © 2012 The American Astronomical Society
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