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

Abstract

Based on the results of applying the extended ADC emission model for low mass X-ray binaries to three Z-track sources: GX 340+0, GX 5-1 and Cyg X-2, we propose an explanation of the Cygnus X-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 starṁ at the onset of flaring and the theoretical critical value at which burning becomes unstable. We thus propose that flaring in the Cygnus X-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.

Anti-Correlated Time Lags in the Z Source Gx 5-1: Possible Evidence for a Truncated Accretion Disk

“…Even the black body temperature is found to be low at 51 Hz compared with the BB temperature at 21 Hz. This decrease in the soft component temperature is similar to the result by Jackson et al (2009) and Balucinska-Church et al (2010) who found that, for GX 5-1 and other Z-track sources, the black body temperature (BB) decreases as the respective source moves from the upper HB to hard apex. We suggest that the low inner disk temperature and high disk normalization at 51 Hz HBO (which is implying the disk is truncated at large radii) -18may be due to an instability caused by the radiation pressure (∝ T 4 ) (Jackson et al 2009).…”

A connection between accretion states and the formation of ultrarelativistic outflows in a neutron star X-ray binary

“…During the transition from HB to NB, a transient radio jet (responsible for radio thin emission) is launched, with a simultaneous decrease in the compact radio jet power. Finally, in the FB, the jet activity is quenched, possibly due to very high accretion rates (see, e.g., Bradshaw et al 2003, but also Bałucińska-Church et al 2010 andChurch et al 2012). As a source moves backwards through the FB and towards the NB, the radio flux increases again.…”

Modeling the Upper kHz QPOs of 4U 1728-34 with X-Ray Reverberation

“…Curiously, we find an unusually high inner disk temperature of 3.0 keV (see Table 2), which is unexpected because the temperature of the thermal component in NS LMXB spectral models (either a blackbody or multicolor disk blackbody, and in many cases both) rarely approaches 3 keV. This is true when modeling reflection features (Cackett et al 2009b(Cackett et al , 2010Sleator et al 2016;Ludlam et al 2017) as well as when reflection is not detected (Bałucińska-Church et al 2010;Church et al 2012Church et al , 2014Homan et al 2018). However, based on the available spectra, we cannot rule out such high disk temperatures, and therefore we include the disk model throughout our analysis as an alternative to our model with a single-temperature blackbody.…”