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تسجيل مستخدم جديدOn the nature of the Cygnus X-2 like Z-track sources
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تأليف
M. Balucinska-Church
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Based on the results of applying the extended ADC emission model for low mass X-ray binaries to three Z-track sources: GX340+0, GX5-1 and CygX-2, we propose an explanation of the CygnusX-2 like Z-track sources. The Normal Branch is dominated by the increasing radiation pressure of the neutron star caused by a mass accretion rate that increases between the soft apex and the hard apex. The radiation pressure continues to increase on the Horizontal Branch becoming several times super-Eddington. We suggest that this disrupts the inner accretion disk and that part of the accretion flow is diverted vertically forming jets which are detected by their radio emission on this part of the Z-track. We thus propose that high radiation pressure is the necessary condition for the launching of jets. On the Flaring Branch there is a large increase in the neutron star blackbody luminosity at constant mass accretion rate indicating an additional energy source on the neutron star. We find that there is good agreement between the mass accretion rate per unit emitting area of the neutron star mdot at the onset of flaring and the theoretical critical value at which burning becomes unstable. We thus propose that flaring in the CygnusX-2 like sources consists of unstable nuclear burning. Correlation of measurements of kilohertz QPO frequencies in all three sources with spectral fitting results leads to the proposal that the upper kHz QPO is an oscillation always taking place at the inner accretion disk edge, the radius of which increases due to disruption of the disk by the high radiation pressure of the neutron star.
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The brightest class of low mass X-ray binary source: the Z-track sources are reviewed specifically with regard to the nature of the three distinct states of the sources. A physical model is presented for the Cygnus X-2 sub-group in which increasing m
ass accretion rate takes place on the Normal Branch resulting in high neutron star temperature and radiation pressure responsible for inner disk disruption and launching of jets. The Flaring Branch consists of unstable nuclear burning on the neutron star. It is shown that the Sco X-1 like sub-group is dominated by almost non-stop flaring consisting of both unstable burning and increase of Mdot, causing higher neutron star temperatures. Finally, results of Atoll source surveys are presented and a model for the nature of the Banana and Island states in these sources is proposed. Motion along the Banana state is caused by variation of Mdot. Measurements of the high energy cut-off of the Comptonized emission E_CO provide the electron temperature T_e of the Comptonizing ADC; above a luminosity of 2x10^37 erg/s E_CO is a few keV and T_e equals the neutron star temperature. At lower luminosities, the cut-off energy rises towards 100 keV showing heating of the corona by an unknown process. This spectral hardening is the cause of the Island state of Atoll sources. The models for Z-track and Atoll sources thus constitute a unified model for low mass X-ray binary sources.
We present results of spectral investigations of the Sco X-1 like Z-track sources Sco X-1, GX 349+2 and GX 17+2 based on Rossi-XTE observations using an extended accretion disk corona model. The results are compared with previous results for the Cyg
X-2 like group: Cyg X-2, GX 340+0 and GX 5-1 and a general model for the Z-track sources proposed. On the normal branch, the Sco-like and Cyg-like sources are similar, the results indicating an increase of mass accretion rate Mdot between soft and hard apex, not as in the standard view that this increases around the Z. In the Cyg-like sources, increasing Mdot causes the neutron star temperature kT to increase from ~1 to ~2 keV. At the lower kT, the radiation pressure is small, but at the higher kT, the emitted flux of the neutron star is several times super-Eddington and the high radiation pressure disrupts the inner disk launching the relativistic jets observed on the upper normal and horizontal branches. In the Sco-like sources, the main physical difference is the high kT of more than 2 keV on all parts of the Z-track suggesting that jets are always possible, even on the flaring branch. The flaring branch in the Cyg-like sources is associated with release of energy on the neutron star consistent with unstable nuclear burning. The Sco-like sources are very different as flaring appears to be a combination of unstable burning and an increase of Mdot which makes flaring much stronger. Analysis of 15 years or RXTE ASM data on all 6 classic Z-track sources shows the high rate and strength of flaring in the Sco-like sources suggesting that continual release of energy heats the neutron star causing the high kT. A Sco X-1 observation with unusually little flaring supports this. GX 17+2 appears to be transitional between the Cyg and Sco-like types. Our results do not support the suggestion that Cyg or Sco-like nature is determined by luminosity.
We review the longterm confusion which has existed over the nature of flaring in the brightest class of low mass X-ray binary: the Z-track sources, specifically in the Cygnus X-2 sub-group. Intensity reductions in the lightcurve produce a branch in c
olour -colour diagrams similar to that of real flares in the Sco X-1 like group, and the nature of this branch was not clear. However, based on observations of Cygnus X-2 in which this dipping/flaring occurred it was proposed that the mass accretion rate in Z-track sources in general increases monotonically along the Z-track towards the Flaring Branch, a standard assumption widely held. It was also suggested that the Cygnus X-2 group have high inclination. Based on recent multi-wavelength observations of Cygnus X-2 we resolve these issues, showing by spectral analysis that the Dipping Branch consists of absorption events in the outer disk, unrelated to the occasional real flaring in the source. Thus motivation for Mdot increasing along the Z from Horizontal - Normal to Flaring Branch is removed, as is the idea that high inclination distinguishes the Cygnus X-2 group. Finally, the observations provide further evidence for the extended nature of the Accretion Disk Corona (ADC), and the correct modelling of the ADC Comptonized emission is crucial to the interpretation of low mass X-ray binary data.
Although the most luminous class of neutron star low mass X-ray binaries, known as Z sources, have been well studied, their behavior is not fully understood. In particular, what causes these sources to trace out the characteristic Z-shaped pattern on
color-color or hardness-intensity diagrams is not well known. By studying the physical properties of the different spectral states of these sources, we may better understand such variability. With that goal in mind, we present a recent NuSTAR observation of the Z source GX 349+2, which spans approximately 2 days, and covers all its spectral states. By creating a hardness-intensity diagram we were able to extract five spectra and trace the change in spectral parameters throughout the Z-track. GX 349+2 shows a strong, broad Fe K$alpha$ line in all states, regardless of the continuum model used. Through modeling of the reflection spectrum and Fe K$alpha$ line we find that in most states the inner disk radius is consistent with remaining unchanged at an average radius of 17.5 $R_g$ or 36.4 km for a canonical 1.4 $M_odot$ neutron star. During the brightest flaring branch, however, the inner disk radius from reflection is not well constrained.
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.
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