A model for the Z-track phenomenon in GX 5-1 and observational evidence for the physical origins of the kHz QPO

We present results of a combined investigation of the spectral and kHz QPO evolution around the Z-track in GX 5-1 based on high-quality RXTE data. The Extended ADC emission model provides very good fits to the spectra, the results pointing clearly to a model for the nature of the Z-track, in agreement with previous results for the similar source GX 340+0. In this model, at the soft apex of the Z-track, the mass accretion rate Mdot is minimum and the neutron star has its lowest temperature; but as the source moves along the normal branch, the luminosity of the Comptonized emission increases, indicating that Mdot increases and the neutron star gets hotter. The measured flux f of the neutron star emission increases by a factor of ten becoming super-Eddington, and we propose that this disrupts the inner disk so forming jets. In flaring, the luminosity of the dominant Comptonized emission from the ADC is constant, while the neutron star emission increases, and we propose for the first time that flaring consists of unstable nuclear burning on the neutron star, and the measured mass accretion rate per unit area mdot at the onset of flaring agrees well with the theoretical critical value at which burning becomes unstable. There is a striking correlation between the frequencies of the kHz QPO and the ratio of the flux to the Eddington value: f/f_Edd, suggesting an explanation of the higher frequency QPO and of its variation along the Z-track. It is well known that a Keplerian orbit in the disk at this frequency corresponds to a position some distance from the neutron star; we propose that the oscillation always occurs at the inner disk edge, which moves radially outwards on the upper normal and horizontal branches as the measured increasing radiation pressure increasingly disrupts the inner disk.Comment: Astronomy and Astrophysics, in pres

Discovery of hard X-ray features around hotspots of Cygnus A

We present results of analysis of a Chandra observation of Cygnus A in which the X-ray hotspots at the ends of the jets are mapped in detail. A hardness map reveals previously unknown structure in the form of outer and inner hard arcs around the hotspots, with hardness significantly enhanced compared with the hotspot central regions. The outer hard arcs may constitute the first detection of the bow shock; the inner hard arcs may reveal where the jets impact on the hotspots. We argue that these features cannot result from electrons radiating by the synchrotron self-Compton process. Instead we consider two possible sources of the hard emission: the outer arcs may be due to thermal radiation of hot intracluster gas compressed at the bow shock. Alternatively, both outer and inner arcs may be due to synchrotron radiation of electrons accelerated in turbulent regions highly perturbed by shocks and shear flows. Comparison of measured hardness ratios with simulations of the hardness ratios resulting from these processes show that it is more diffcult to explain the observations with a thermal model. Although we cannot rule out a thermal model, we argue in favour of the non-thermal explanation. The hard regions in the secondary hotspots suggest that jet activity is still powering these hotspots.Comment: MNRAS in press; 5 pages, 3 figures (2 figures in colour in jpeg format should be printed separately

The Gettier Problem

In this chapter, we will explore the luck at issue in Gettier-styled counterexamples and the subsequent problem it poses to any viable reductive analysis of knowledge. In the 1st section, we will consider the specific species of luck that is at issue in Gettier counterexamples, then, in the next section, I will briefly sketch a diagnosis of the Gettier Problem and try to explain why the relevant species of luck has proven to be extremely difficult to avoid. And finally, I will consider a prominent objection to the proposed diagnosis of the Problem