Evolution of the truncated disc and inner hot-flow of GX 339-4

Abstract

Aims. We study the changes in geometry of the truncated disc and the inner hot-flow of GX 339-4 by analysing the power spectral density (PSD) extracted from six XMM-Newton observations taken at the very end of an outburst. Methods. We developed a theoretical model of the PSD of GX 339-4 in the 0.3-0.7 keV (thermal-reverberation-dominated) and 0.7-1.5 keV (disc-continuum-dominated) energy bands. The model assumes the standard accretion disc to be truncated at a specific radius, inside of which are two distinct hot-flow zones: one spectrally soft and the other spectrally hard. The effects of disc fluctuations and thermal reverberation are taken into account. Results. This model successfully produces the traditional bumpy PSD profiles and provides good fits to the GX 339-4 data. The truncation radius is found to increase from rtrc 10 to 55rg as the source luminosity decreases, confirming that the truncation radius can be characterized as a function of luminosity. Keeping in mind the large uncertainty in previous measurements of the truncation radius, our values are larger than some obtained from spectroscopic analysis, but smaller than those implied by reverberation lag analysis. Furthermore, the size of two inner hot-flow zones that are spectrally hard and spectrally soft also increases from 5 to 27rg and from 3 to 26rg, respectively, as the flux decreases. We find that the radial range of the inner hard zone is always larger than the range of the soft hot-flow zone, but by a comparatively small factor of 1.1-2.2