Spectral Investigations of the nature of the Sco X-1 like 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.Comment: Astronomy and Astrophysics in press; 21 pages, 13 figure

On the nature of the Cygnus X-2 like Z-track sources

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.Comment: Astronomy and Astrophysics, in pres

Low-Mass X-ray Binaries and Globular Clusters in Early-Type Galaxies. I. Chandra Observations

We present a Chandra survey of LMXBs in 24 early-type galaxies. Correcting for detection incompleteness, the X-ray luminosity function (XLF) of each galaxy is consistent with a powerlaw with negative logarithmic differential slope, beta~2.0. However, beta strongly correlates with incompleteness, indicating the XLF flattens at low-Lx. The composite XLF is well-fitted by a powerlaw with a break at 2.21(+0.65,-0.56)E38 erg/s and beta=1.40(+0.10,-0.13) and 2.84(+0.39,-0.30) below and above it, respectively. The break is close to the Eddington limit for a 1.4Msun neutron-star, but the XLF shape rules out its representing the division between neutron-star and black-hole systems. Although the XLFs are similar, we find evidence of some variation between galaxies. The high-Lx XLF slope does not correlate with age, but may correlate with [alpha/Fe]. Considering only LMXBs with Lx>1E37 erg/s, matching the LMXBs with globular clusters (GCs) identified in HST observations of 19 of the galaxies, we find the probability a GC hosts an LMXB is proportional to LGC^alpha ZFe^gamma} where alpha=1.01+/-0.19 and gamma=0.33+/-0.11. Correcting for GC luminosity and colour effects, and detection incompleteness, we find no evidence that the fraction of LMXBs with Lx>1e37 erg/s in GCs (40%), or the fraction of GCs hosting LMXBs (~6.5%) varies between galaxies. The spatial distribution of LMXBs resembles that of GCs, and the specific frequency of LMXBs is proportional to the GC specific luminosity, consistent with the hypothesis that all LMXBs form in GCs. If the LMXB lifetime is tau and the duty cycle is Fd, our results imply ~1.5 (tau/1E8 yr)^-1 /Fd LMXBs are formed per Gyr per GC and we place an upper limit of 1 active LMXB in the field per 3.4E9Lsun of V-band luminosity.Comment: 24 pages, 17 figures and 6 tables. Accepted for publication in the Astrophysical Journal. Expanded discussion and various minor revisions to improve robustness of results. Conclusions unchange

The Distribution of X-ray Dips with Orbital Phase in Cygnus X-1

We present results of a comprehensive study of the distribution of absorption dips with orbital phase in Cygnus X-1. Firstly, the distribution was obtained using archival data from all major X-ray observatories and corrected for the selection effect that phase zero (superior conjunction of the black hole) has been preferentially observed. Dip occurrence was seen to vary strongly with orbital phase \phi, with a peak at \phi ~ 0.95, i.e. was not symmetric about phase zero. Secondly, the RXTE ASM has provided continuous coverage of the Low State of Cygnus X-1 since Sept. 1996, and we have selected dip data based on increases in hardness ratio. The distribution, with much increased numbers of dip events, confirms that the peak is at \phi ~ 0.95, and we report the discovery of a second peak at \phi ~ 0.6. We attribute this peak to absorption in an accretion stream from the companion star HDE 226868. We have estimated the ionization parameter at different positions showing that radiative acceleration of the wind is suppressed by photoionization in particular regions in the binary system. To obtain the variation of column density with phase, we make estimates of neutral wind density for the extreme cases that acceleration of the wind is totally suppressed, or not suppressed at all. An accurate description will lie between these extremes. In each case, a strong variation of column density with orbital phase resulted, similar to the variation of dip occurrence. This provides evidence that formation of the blobs in the wind which lead to absorption dips depends on the density of the neutral component in the wind, suggesting possible mechanisms for blob growth.Comment: 9 pages, Latex, 7 ps figures. accepted by MNRA