Horizontal Branch Oscillations Research Articles

AstroSat and NICER timing view of the Z−type neutron star X-ray binary GX 340 + 0

ABSTRACT The timing properties of the Z-type low-mass X-ray binaries provide insights into the emission components involved in producing the unique Z-shaped track in the hardness–intensity diagrams of these sources. In this work, we investigate the AstroSat and NICER observations of the GX 340 + 0 covering the complete ‘Z’-track from the horizontal branch (HB) to the extended flaring branch (EFB). For the first time, we present the Z-track as seen in soft X-rays using the AstroSat/Soft X-ray Telescope and NICER (the soft colour is defined as a ratio of 3–6 to 0.5–3 keV). The shape of the track is distinctly different in soft X-rays, strongly suggesting the presence of additional components active in soft X-rays. The detailed timing analysis revealed significant quasi-periodic oscillation throughout the HB and the normal branch (NB) using large area X-ray proportional counter and the first NICER detection of 33.1 ± 1.1 Hz HB oscillation (HBO) in 3–6 keV. The oscillations at the HB/NB vertex are observed to have higher frequencies (41–52 Hz) than the HBOs (16–31 Hz) and NB oscillations (6.2–8 Hz) but significantly lower rms (∼1.6 per cent). The HBO is also limited to the energy range 3–20 keV, indicating an association of HBO origin with the non-thermal component. It is also supported by earlier studies that found the strongest X-ray polarization during HB.

Spectral and temporal features of GX 13+1 as revealed by AstroSat

ABSTRACT GX 13+1, a neutron star low-mass X-ray binary that exhibits the properties of both atoll and Z sources, is studied using data from Soft X-ray Telescope and Large Area X-ray Proportional Counter (LAXPC) onboard AstroSat. The source traces a ν shaped track in its hardness-intensity diagram (HID). Spectral modelling of the data in the 0.7–30.0 keV energy range, with the model – $\tt {constant}$ × $\tt {tbabs}$ × $\tt {thcomp}$ × $\tt {bbodyrad}$ + $\tt {relxillNS}$, yields orbital inclination angle (θ) of 77$\substack{+10\\-8}$°. Flux resolved spectral analysis reveals the ν shaped pattern in the plots of spectral parameters kTe, kTbb, and Γ versus Fbol, closely resembling the pattern traced in LAXPC HID. This indicates changes in the spectral properties of the corona and the boundary layer/accretion disc. Assuming that the accretion disc truncates at the Alfvén radius, the upper limit of the magnetic field strength (B) at the poles of neutron star in GX 13+1 is calculated to be 5.10 × 108 G (for kA = 1 and η = 0.1), which is close to that of atoll sources. Furthermore, thickness of the boundary layer is estimated to be 5.70 km, which results in the neutron star radius value of ≲14.50 km. Quasi-periodic oscillations (QPOs) at 56 ± 4 and 54 ± 4 Hz are detected in Regions D and E of HID, respectively. The frequencies of these QPOs are similar to the characteristic frequency of horizontal branch oscillation and these do not exhibit a positive correlation with mass accretion rate.

Anticorrelated lags in a neutron star Z source GX 5-1: AstroSat’s View

ABSTRACT We report the cross-correlation function studies of a neutron star low-mass X-ray binary, a Z source GX 5-1, using Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counter (LAXPC) energy bands onboard AstroSat. For the first time, we report the lag between soft (0.8–2.0 keV, SXT) and hard X-ray energy bands (10–20 and 16–40 keV, LAXPC) in GX 5-1 and detected lags of the order of a few tens to hundreds of seconds in the horizontal branch. We interpreted them as the readjustment time-scale of the inner region of the accretion disc. We used various two components and three-component spectral models to unfold the spectra and observed the changes in soft and hard component fluxes that were exhibiting horizontal branch oscillation variations. It was observed that the bbody component assumed to be originating from the boundary layer over the NS and was also found to vary along with the HBO variation where lags were detected. We constrained the size of the comptonizing region of the order 15–55 km, assuming that lags were due to variation in the size of the corona. We noticed a similar size of the comptonizing region after employing other models and suggest that the overall size of corona must be of the order of a few tens of km to explain the lags, HBO variation, and respective spectral variations. In a case study, it was noted that the BL size increases as GX 5-1 vary from the top of the HB to the upper vertex.

Very low-frequency oscillations from the 11 Hz pulsar in Terzan 5: frame dragging back on the table.

ABSTRACT We present a re-analysis of 47 Rossi X-ray Timing Explorer observations of the 11 Hz accreting pulsar IGR J17480-2446 in Terzan 5 during its 2010 outburst. We studied the fast-time variability properties of the source and searched for quasi-periodic oscillations (QPOs) in a large-frequency range. General relativity predicts that frame dragging occurs in the vicinity of a spinning compact object and induces the precession of matter orbiting said object. The relativistic precession model predicts that this frame dragging can be observed as QPOs with a characteristic frequency in the light curves of accreting compact objects. Such QPOs have historically been classified as horizontal branch oscillations in neutron star (NS) systems, and for a NS spinning at 11 Hz, these oscillations are expected at frequencies below 1 Hz. However, previous studies of IGR J17480-2446 have classified QPOs at 35–50 Hz as horizontal branch oscillations, thus casting doubts on the frame-dragging nature of such QPOs. Here, we report the detection of seven very low-frequency QPOs, previously undetected, with centroid frequencies below 0.3 Hz, and which can be ascribed to frame dragging. We also discuss the possible nature of the QPOs detected at 35–50 Hz in this alternative scenario.

Insight-HXMT study of the timing properties of Sco X-1

We present a detailed timing study of the brightest persistent X-ray source Sco X-1 using the data collected by the Hard X-ray Modulation Telescope (Insight-HXMT) from July 2017 to August 2018. A complete Z-track hardness-intensity diagram (HID) is obtained. The normal branch oscillations (NBOs) at ∼ 6 Hz in the lower part of the normal branch (NB) and the flare branch oscillations (FBOs) at ∼ 16 Hz in the beginning part of the flaring branch (FB) are found in observations with the Low Energy X-ray Telescope (LE) and the Medium Energy X-ray Telescope (ME) of Insight-HXMT, while the horizontal branch oscillations (HBOs) at ∼ 40 Hz and the kilohertz quasi-periodic oscillations (kHz QPOs) at ∼ 800 Hz are found simultaneously above 20 keV for the first time on the horizontal branch (HB) by the High Energy X-ray Telescope (HE) and ME. We find that for all types of the observed QPOs, the centroid frequencies are independent of energy, while the root mean square (rms) increases with energy; the centroid frequencies of both the HBOs and kHz QPOs increase along the Z-track from the top to the bottom of the HB; and the NBOs show soft phase lags increasing with energy. A continuous QPO transition from the FB to NB in ∼ 200 s are also detected. Our results indicate that the non-thermal emission is the origin of all types of QPOs, the innermost region of the accretion disk is non-thermal in nature, and the corona is nonhomogeneous geometrically.

Neutron star QPOs from oscillating, precessing hot, thick flow

ABSTRACT Across black hole (BH) and neutron star (NS) low-mass X-ray binaries (LMXBs), there appears to be some correlation between certain high- and low-frequency quasi-periodic oscillations (QPOs). In a previous paper, we showed that for BH LMXBs, this could be explained by the simultaneous oscillation and precession of a hot, thick, torus-like corona. In the current work, we extend this idea to NS LMXBs by associating the horizontal branch oscillations (HBOs) with precession and the upper-kiloHertz (ukHz) QPO with vertical epicyclic motion. For the Atoll source 4U 1608-52, the model can match many distinct, simultaneous observations of the HBO and ukHz QPO by varying the inner and outer radius of the torus, while maintaining fixed values for the mass (MNS) and spin (a*) of the NS. The best-fitting values are MNS = 1.38 ± 0.03 M⊙ and a* = 0.325 ± 0.005. By combining these constraints with the measured spin frequency, we are able to obtain an estimate for the moment of inertia of INS = 1.40 ± 0.02 × 1045 g cm2, which places constraints on the equation of state. The model is unable to fit the lower-kHz QPO, but evidence suggests that QPO may be associated with the boundary layer between the accretion flow and the NS surface, which is not treated in this work.

Low Frequency Quasi-periodic Oscillations in the High-eccentric LMXB Cir X-1: Extending the WK Correlation for Z Sources

Abstract Using archival Rossi X-ray Timing Explorer (RXTE) data, we studied the low-frequency quasi-periodic oscillations (LFQPOs) in the neutron star low-mass X-ray binary (LMXB) Cir X-1 and examined their contribution to frequency–frequency correlations for Z sources. We also studied the orbital phase effects on the LFQPO properties and found them to be phase independent. Comparing LFQPO frequencies in different classes of LMXBs, we found that systems that show both Z and atoll states form a common track with atoll/BH sources in the so-called WK correlation, while persistent Z systems are offset by a factor of about two. We found that neither source luminosity nor mass accretion rate is related to the shift of persistent Z systems. We discuss the possibility of a misidentification of fundamental frequency for horizontal branch oscillations from persistent Z systems and interpreted the oscillations in terms of models based on relativistic precession.

CORRELATIONS IN HORIZONTAL BRANCH OSCILLATIONS AND BREAK COMPONENTS IN XTE J1701-462 AND GX 17+2

We studied the horizontal branch oscillations (HBO) and the band-limited components observed in the power spectra of the transient neutron star low-mass X-ray binary XTE J1701-462 and the persistent "Sco-like" Z source GX 17+2. These two components were studied based on the state-resolved spectra. We found that the frequencies of XTE J1701-462 lie on the known correlations (WK and PBK), showing consistency with other types of X-ray binaries (black holes, atoll sources and millisecond X-ray pulsars). However, GX 17+2 is shifted from the WK correlation like other typical Z sources. We suggest that the WK/PBK main track forms a boundary which separates persistent sources from transient sources. The characteristic frequencies of break and HBO are independent of accretion rate in both sources, although it depends on spectral models. We also report the energy dependence of the HBO and break frequencies in XTE J1701-462 and how the temporal properties change with spectral state in XTE J1701-462 and GX 17+2. We studied the correlation between rms at the break and the HBO frequency. We suggest that HBO and break components for both sources probably arise from a similar physical mechanism: Comptonization emission from the corona. These two components could be caused by same kind of oscillation in a corona who with uneven density, and they could be generated from different areas of corona. We further suggest that different proportions of the Comptonization component in the total flux cause the different distribution between GX 17+2 and XTE J1701-462 in the $rms_{\rm{break}}$-$rms_{\rm{HBO}}$ diagram.

THE ACCRETION RATE INDEPENDENCE OF HORIZONTAL BRANCH OSCILLATION IN XTE J1701-462

We study the temporal and energy spectral properties of the unique neutron star low-mass X-ray binary XTE J1701-462. After assuming the HB/NB vertex as a reference position of accretion rate, the horizontal branch oscillation (HBO) of the HB/NB vertex is roughly 50 Hz. It indicates that the HBO is independent with the accretion rate or the source intensity. The spectral analysis shows $R_{\rm{in}}\propto\dot{M}_{\rm{Disk}}^{2.9\pm0.09}$ in the HB/NB vertex and $R_{\rm{in}}\propto\dot{M}_{\rm{Disk}}^{1.7\pm0.06}$ in the NB/FB vertex, which implies that different accretion rate may be produced in the HB/NB vertex and the NB/FB vertex. The Comptonization component could be fitted by constrained broken power law (CBPL) or nthComp. Different with GX 17+2, the frequencies of HBO positively correlate with the inner disk radius, which contradict with the prediction of Lense-Thirring precession model. XTE J1701-462, both in the Cyg-like phase and in the Sco-like phase, follows a positive correlation between the break frequency of broad band noise and the characteristic frequency of HBO, which is called the W-K relation. An anticorrelation between the frequency of HBO and photon energy is observed. Moreover, the rms of HBO increases with photon energy till ~10 keV. We discuss the possible origin of HBO from corona in XTE J1701-462.

THE CROSS SPECTRAL TIME LAG EVOLUTION ALONG BRANCHES IN XTE J1701-462

We investigate the cross spectrum of XTE J1701-462 in various types of Neutron Star-Low Mass X-ray Binary subclasses, during its 2006-2007 outburst. We analyze the relation between the time lags and temporal variabilities. We find that the hard time lags accompany with the horizontal branch oscillations (HBOs) and the soft time lags dominate the noise in the low frequency range 0.1-10 Hz on HB. In the Cyg-like phase, the time lags decrease on the middle normal branch (NB) from HB/NB vertex to NB/FB vertex, whereas the time lags are roughly invariant in the Sco-like source. We discuss that the Compton upscattering by the corona introduces the soft lag in low frequency noise. We interpret that the variation of the Comptonization component from the disk emission lead to the HBOs' time lags evolution along the Z tracks. We also report the rms amplitude spectrum and phase lag spectrum for the NBO. The $\sim$ $160^{\circ}$ phase lag is found. We present that the rms amplitude of both the Cyg-like and the Sco-like NBO linearly increase with the photon energy in low energy bands, and it will drop in the highest energy band.

An Attempt to Describe Frequency-Correlations among kHz QPOs and HBOs by Two-Armed Vertical p-Mode Oscillations: Case of No Magnetic Field

The trapping of two-armed ($ m$$ =$ 2) vertical p-mode oscillations in relativistic disks is examined. The disks are assumed to be isothermal in the vertical direction, but are truncated at a certain height by the presence of corona. The same issues were examined in a previous paper (Kato 2012a). In this paper, unlike the previous paper, however, we do not use the approximation that the oscillations are nearly vertical, but limit to a simpler case of no magnetic field. As in the previous paper, the results suggest that the two basic oscillation modes [both are the fundamental ($ n$$ =$ 1) in the vertical direction, but in the horizontal direction one is the fundamental ($ n_{\rm r}$$ =$ 0) and the other the first overtone ($ n_{\rm r}$$ =$ 1)] correspond to the twin kHz QPOs. Second, the oscillation mode, which is the first overtone ($ n$$ =$ 2) in the vertical direction and the fundamental in the horizontal direction ($ n_{\rm r}$$ =$ 0), will correspond to the horizontal branch oscillation (HBO) of Z-sources. The latter suggests that the horizontal branch of Z-sources is a sequence of temperature change in disks whose vertical thickness is strongly terminated. The temperature increases leftward along the sequence from the apex between the normal and horizontal branches.

LOW-FREQUENCY QUASI-PERIODIC OSCILLATION FROM THE 11 Hz ACCRETING PULSAR IN TERZAN 5: NOT FRAME DRAGGING

We report on 6 RXTE observations taken during the 2010 outburst of the 11 Hz accreting pulsar IGR J17480-2446 located in the globular cluster Terzan 5. During these observations we find power spectra which resemble those seen in Z-type high-luminosity neutron star low-mass X-ray binaries, with a quasi-periodic oscillation (QPO) in the 35-50 Hz range simultaneous with a kHz QPO and broad band noise. Using well known frequency-frequency correlations, we identify the 35-50 Hz QPOs as the horizontal branch oscillations (HBO), which were previously suggested to be due to Lense-Thirring precession. As IGR J17480-2446 spins more than an order of magnitude more slowly than any of the other neutron stars where these QPOs were found, this QPO can not be explained by frame dragging. By extension, this casts doubt on the Lense-Thirring precession model for other low-frequency QPOs in neutron-star and perhaps even black-hole systems.

Trapped, Two-Armed, Nearly Vertical Oscillations in Disks with Toroidal Magnetic Fields II: Effects of Finite Thickness

We examine the radial trapping of two-armed ($ m$$ =$ 2) vertical p-mode oscillations in geometrically thin relativistic disks that are vertically isothermal, but terminated at a certain height by the presence of hot and low-density corona. The disks are assumed to be subject to toroidal magnetic fields. The oscillations are classified by $ n$ , a number related to the node number of oscillations in the vertical direction, and starting from $ n$$ =$ 1. In modes with $ n$$ =$ 1, the frequencies of trapped oscillations depend little on the height of termination, but in modes with $ n$$ =$ 2, 3, $ \cdots$ the frequencies decrease and the radial extentions of the trapped region become wide, as the termination height decreases. This study is a preparation to examine whether these oscillations can describe kilo-hertz quasi-periodic oscillations (kHz QPOs), horizontal branch oscillations (HBOs), and their correlations.

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

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.

An Attempt to Describe Frequency Correlations among kHz QPOs and HBOs by Two-Armed Nearly Vertical Oscillations

We examine whether the two-armed ($m$$=$ 2) vertical p-mode oscillations trapped in the innermost region of magnetized accretion disks with finite disk thickness can describe kHz quasi-periodic oscillations (QPOs) and horizontal branch oscillations (HBOs) in low-mass X-ray binaries (LMXBs). First, we derive the frequency–frequency correlation of the two basic oscillations (both are fundamental modes in the vertical direction, but one is the fundamental and the other the first overtone in the radial direction), and compare it with the observed frequency correlation of twin kHz QPOs. Results show that the calculated frequency correlation can well describe the observed frequency one with reasonable values of the parameters. Second, we examine whether the observed frequency correlation between kHz QPOs and HBO can be described by regarding HBO as the first overtone oscillation in the vertical direction (and the fundamental in the radial direction). The results suggest that (i) the innermost parts of disks on the horizontal branch are strongly diminished in their vertical thickness (presumably by hot coronae) and (ii) the branch is roughly a sequence of variations of magnetic fields or disk temperature.

ANTI-CORRELATED TIME LAGS IN THE Z SOURCE GX 5-1: POSSIBLE EVIDENCE FOR A TRUNCATED ACCRETION DISK

We investigate the nature of the inner accretion disk in the neutron star source GX 5-1 by making a detailed study of time lags between X-rays of different energies. Using the cross-correlation analysis, we found anti-correlated hard and soft time lags of the order of a few tens to a few hundred seconds and the corresponding intensity states were mostly the horizontal branch (HB) and upper normal branch (NB). The model independent and dependent spectral analysis showed that during these time lags the structure of accretion disk significantly varied. Both eastern and western approaches were used to unfold the X-ray continuum and systematic changes were observed in soft and hard spectral components. These changes along with a systematic shift in the frequency of quasi-periodic oscillations (QPOs) made it substantially evident that the geometry of the accretion disk is truncated. Simultaneous energy spectral and power density spectral study shows that the production of the horizontal branch oscillations (HBOs) are closely related to the Comptonizing region rather than the disk component in the accretion disk. We found that as the HBO frequency decreases from the hard apex to upper HB, the disk temperature increases along with an increase in the coronal temperature which is in sharp contrast with the changes found in black hole binaries where the decrease in QPO frequency is accompanied by a decrease in the disk temperature and a simultaneous increase in the coronal temperature. We discuss the results in the context of re-condensation of coronal material in the inner region of the disk.

COUPLED HBO AND NBO VARIATIONS IN THE Z SOURCE GX 5-1: INNER ACCRETION DISK AS THE LOCATION OF QPOs

The simultaneous and coupled evolution of horizontal branch oscillation (HBO) and normal branch oscillation (NBO) in Z-type sources suggests that the production of HBO is connected to NBO and is caused by changes in the physical/radiative properties of the inner accretion disk, although there is a lack of substantial spectral evidence to support this. In this {\it Letter}, we present the results of an analysis of a RXTE observation of a Z source GX~5-1, where the 6 Hz NBO is simultaneously detected along with a HBO at 51 Hz. The variations in the intensity and the associated power density spectrum indicate that the HBO and NBO are strongly coupled, originating from the same location in the inner accretion disk. The absence of HBO and NBO in the lower energy bands, an increase in the rms amplitude with energy and a smooth transition among them suggest that they are produced in the hot inner regions of the accretion disk. Based on a spectral analysis, we found a signature of changing or physically modified inner disk front during the coupled HBO and NBO evolution. We explore the various models to explain the observed phenomenon and propose that the NBO is affiliated to the oscillations in the thick/puffed-up inner region of the accretion disk.

Interpretations for Low‐ and High‐Frequency QPO Correlations of X‐Ray Sources among White Dwarfs, Neutron Stars, and Black Holes

ABSTRACTAn empirical linear relation is found to exist between the high and low frequencies (νhigh, νlow) of quasi‐periodic oscillations (QPOs) for black hole candidates (BHCs), neutron stars (NSs), and white dwarfs (WDs) in binary systems, spanning 5 orders of magnitude in frequency. For the NS Z (atoll) sources, νhigh and νlow are identified as the lower kHz QPO frequency and horizontal‐branch oscillation (HBO) frequency νHBO (broad noise components); for the BHCs and low‐luminosity NSs, they are the QPOs and broad noise components at frequencies between 1 and 10 Hz; for WDs, they are the “dwarf nova oscillations” (DNOs) and QPOs of cataclysmic variables (CVs). To interpret this relation, our model ascribes νhigh to the Alfvén wave oscillation frequency at a preferred radius, and νlow to the same mechanism at another radius. We can then obtain νlow = 0.08νhigh and the relation between the upper kHz QPO frequency ν2 and the HBO frequency, νHBO≃(56 Hz)(ν2/kHz)2, which are in accordance with the observed empirical relations. Furthermore, some implications of the model are discussed, including why QPO frequencies of WDs and NSs span 5 orders of magnitude.

Coupling between the 45 Hz Horizontal-Branch Oscillation and the Normal-Branch Oscillation in Scorpius X-1

The observations of the bright persistent neutron star low-mass X-ray binary (LMXB) Sco X-1 performed with the Rossi X-Ray Timing Explorer (RXTE) show a ~6 Hz normal-branch oscillation (NBO), a ~45 Hz horizontal-branch oscillation (HBO), and twin kilohertz quasi-periodic oscillations (kHz QPOs) on its normal branch simultaneously. We have found that the fractional amplitude of the HBO corresponding to the NBO phase of high flux is 1.1%, while that of the NBO phase of low flux is undetectable, with a 3 σ upper limit of 0.4%, implying that the HBO strength varies with the NBO phase in a manner opposite that of the lower kHz QPO previously found, and suggests that the condition for the generation of the HBO is met when the NBO flux is high. The 6 Hz NBO in Sco X-1 connects the 45 Hz HBO and the twin kHz QPO together, showing a unique picture indicating a coupling between the QPOs, which has never been observed in other neutron star LMXBs. We discuss the implications for current models of the 45 Hz HBO, the 6 Hz NBO, and the twin kHz QPOs.

Rossi X‐Ray Timing ExplorerObservations of the First Transient Z Source XTE J1701−462: Shedding New Light on Mass Accretion in Luminous Neutron Star X‐Ray Binaries

We report on the first 10 weeks of RXTE observations of the X-ray transient XTE J1701-462 and conclude that it had all the characteristics of the neutron star Z sources, i.e., the brightest persistent neutron star low-mass X-ray binaries. These include the typical Z-shaped tracks traced out in X-ray color diagrams and the variability components detected in the power spectra, such as kHz QPOs and normal and horizontal branch oscillations. XTE J1701-462 is the first transient Z source and provides unique insights into mass accretion rate () and luminosity dependencies in neutron star X-ray binaries. As its overall luminosity decreased, we observed a switch between two types of Z source behavior, with the branches of the Z track changing their shape and/or orientation. We interpret this as an extreme case of the more moderate long-term changes seen in the persistent Z sources and suggest that they result from changes in . We also suggest that the Cyg-like Z sources (Cyg X-2, GX 5-1, and GX 340+0) are substantially more luminous (>50%) than the Sco-like Z sources (Sco X-1, GX 17+2, and GX 349+2). Adopting a possible explanation for the behavior of kHz QPOs, which involves a prompt as well as a filtered response to changes in , we further propose that changes in can explain both movement along the Z track and changes in the shape of the Z track. We discuss some consequences of this and consider the possibility that the branches of the Z will smoothly evolve into the branches observed in X-ray color diagrams of the less luminous atoll sources, although not in a way that was previously suggested.