RXTEObservations of the Neutron Star Low‐Mass X‐Ray Binary GX 17+2: Correlated X‐Ray Spectral and Timing Behavior

We have analyzed ~600 ks of Rossi X-ray Timing Explorer data of the neutronstar low-mass X-ray binary and Z source GX 17+2. A study was performed of theproperties of the noise components and quasi-periodic oscillations (QPOs) as afunction of the spectral properties, with the main goal to study the relationbetween the frequencies of the horizontal branch and upper kHz QPOs. It wasfound that when the upper kHz QPO frequency is below 1030 Hz these frequenciescorrelate, whereas above 1030 Hz they anti-correlate. GX 17+2 is the firstsource in which this is observed. We also found that the frequency differenceof the high frequency QPOs was not constant and that the quality factors (Qvalues) of the HBO, its second harmonic, and the kHz QPOs are similar, and varyalmost hand in hand by a factor of more than three. Observations of the normalbranch oscillations during two type I X-ray bursts showed that their absoluteamplitude decreased as the flux from the neutron star became stronger. Wediscuss these and other findings in terms of models that have been proposed forthese phenomena. We also compare the behavior of GX 17+2 and other Z sourceswith that of black hole sources and consider the possibility that the massaccretion rate might not be driving force behind all spectral and variabilitychanges.Comment: 35 pages, including 14 figures. Accepted for publication in ApJ. Revised discussion, one new figure, and some minor figure changes with respect to old versio