An explanation of the Z-track sources

Abstract

We present an explanation of the Z-track phenomenon based on spectral fitting of RXTE observations of GX340+0 using the emission model previously shown to describe the dipping LMXB. In our Z-track model, the soft apex is a quiescent state of the source with lowest luminosity. Moving away from this point by ascending the normal branch the strongly increasing luminosity of the Accretion Disc Corona (ADC) Comptonized emission L_ADC provides substantial evidence for a large increase of mass accretion rate Mdot. There are major changes in the neutron star blackbody emission, kT increasing to high values, the blackbody radius R_BB decreasing, these changes continuing monotonically on both normal and horizontal branches. The blackbody flux increases by a factor of ten to three times the Eddington flux so that the physics of the horizontal branch is dominated by the high radiation pressure of the neutron star, which we propose disrupts the inner disc, and an increase of column density is detected. We further propose that the very strong radiation pressure is responsible for the launching of the jets detected in radio on the horizontal branch. On the flaring branch, we find that L_ADC is constant, suggesting no change in Mdot so that flaring must consist of unstable nuclear burning. At the soft apex, the mass accretion rate per unit area on the neutron star m_dot is minimum for the horizontal and normal branches and about equal to the theoretical upper limit for unstable burning. Thus it is possible that unstable burning begins as soon as the source arrives at this position, the onset being consistent with theory. The large increase in R_BB in flaring is reminiscent of radius expansion in X-ray bursts. Finally, in our model, Mdot does not increase monotonically along the Z-track as often previously thought.

14 pages, 8 figures, accepted for publication in Astronomy and Astrophysics

References in corpus (24)

• Jets in neutron star X-ray binaries: a comparison with black holes
• The rp Process Ashes from Stable Nuclear Burning on an Accreting Neutron Star
• The Discovery of a State Dependent Hard Tail in the X-ray Spectrum of the Luminous Z-source GX 17+2
• The radio luminosity of persistent X-ray binaries
• X-ray bursts at extreme mass accretion rates from GX 17+2
• Testing Comptonization models using BeppoSAX observations of Seyfert 1 galaxies
• Detection of a Hard Tail in the X-ray Spectrum of the Z Source GX 349+2
• On the Spectral Evolution of Cygnus X-2 along its Color-Color Diagram
• Measurements of accretion disc corona size in LMXB: consequences for Comptonization and LMXB models
• The power spectral properties of the Z-source GX 340+0
• Results of a LMXB survey: variation in the height of the neutron star blackbody emission region
• X-Ray Spectra of Z Sources
• GX 17+2: X-ray spectral and timing behaviour of a bursting Z source
• Simple Photoelectric Absorption during Dipping in the ASCA Observation of XB1916-053
• Discovery of kilohertz quasi-periodic oscillations in the Z source GX 340+0
• The X-ray Spectrum of Cyg X-2
• The Ephemeris and Dipping Spectral Behavior of X1624-490
• X-ray spectral evolution of low-mass X-ray binary GX 349+2
• Physical changes during Z-track movement in Sco X-1 on the flaring branch
• Discovery of absorption features of the ADC and systematic acceleration of the X-ray burst rate in XB 1323-619
• A study of the spectral evolution during dipping in XB 1323-619 with Rossi-XTE and BeppoSAX
• Neutron star blackbody contraction during flaring in X1624-490
• The emission regions in X-ray binaries: dipping as a diagnostic
• A comparison of neutron star blackbody luminosities in LMXB with the theory of accretion flow spreading on the stellar surface

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• On the nature of the Cygnus X-2 like Z-track sources
• X-ray spectral and timing behavior of Scorpius X-1. Spectral hardening during the flaring branch
• A model for the Z-track phenomenon in GX 5-1 and observational evidence for the physical origins of the kHz QPO
• Absence of reflection features in NuSTAR spectra of the luminous neutron star X-ray binary GX 5-1
• Stability of the photon indices in Z-source GX 340+0 for spectral states
• Radio and X-ray observations of jet ejection in Cygnus X-2
• X-ray spectral evolution of the extragalactic Z-source, LMC X-2
• An Attempt to Describe Frequency Correlations among kHz QPOs and HBOs by Two-Armed Nearly Vertical Oscillations
• Dipping in Cygnus X-2 in a multi-wavelength campaign due to absorption of extended ADC emission
• Constraining the Coronal Heights and Readjustment Velocities Based on the Detection of a Few Hundred Seconds Delays in the Z Source GX 17+2
• Kepler K2 Observations of Sco X-1: Orbital Modulations and Correlations with Fermi GBM and MAXI
• Neutral absorber dips in the periodic burster LMXB XB 1323-619 from Suzaku
• AstroSat view of the neutron star low-mass X-ray binary GX 340+0
• Cross-correlations between soft and hard light curves depending on luminosity in the transient neutron star XTE J1701-462
• Thermonuclear X-ray bursts detected in Cyg X-2 using AstroSat/LAXPC
• Spectral analysis of LMC X-2 with XMM/Newton: unveiling the emission process in the extragalactic Z-source
• An Attempt to Describe Frequency-Correlations among kHz QPOs and HBOs by Two-Armed Vertical p-mode Oscillations: Case of No Magnetic Field
• Spectral and Timing evolution of GX 340+0 along its Z-track
• Swift monitoring of Cygnus X-2: investigating the NUV-X-ray connection
• Broadband spectro-temporal investigation of neutron star low-mass X-ray binary GX 349+2
• The cross-correlation analysis in Z source GX 349+2
• The hard X-ray emission along the "Z" track in GX 17+2
• Insight-HXMT Detections of Hard X-ray Tails in Scorpius X-1
• The accretion rate independence of horizontal branch oscillation in XTE J1701-462
• Investigating the coronal structure by studying time lags in the Atoll source 4U 1705-44 using AstroSat
• X-ray spectropolarimetric characterization of the Z-source GX 340+0 in the normal branch
• Spectro-temporal evolution of 4U 1702-429 using AstroSat-NICER
• The rms-flux relations in different branches in Cyg X-2
• Links Between Optical and X-ray Light in Cygnus X-2
• A NICER and AstroSat view of the neutron star low-mass X-ray binary 1A 1246-588