Coronal-temporal correlations in GX 339-4: hysteresis, possible reflection changes and implications for advection-dominated accretion flows

Abstract

We present spectral fits and timing analysis of <it>Rossi X-ray Timing Explorer</it> observations of GX 339−4. These observations were carried out over a span of more than two years and encompassed both the soft/high and hard/low states. Two observations were simultaneous with <it>Advanced Satellite for Cosmology and Astrophysics</it> observations. Hysteresis in the soft–hard state transition is observed. The hard state exhibits a possible anticorrelation between coronal compactness (i.e. spectral hardness) and the covering fraction of cold, reflecting material. The correlation between ‘reflection fraction’ and soft X-ray flux, however, appears to be more universal. Furthermore, low-flux, hard-state observations – taken over a decline into quiescence – show that the Fe line, independent of ‘reflection fraction’, remains broad and at a roughly constant equivalent width, counter to expectations from advection-dominated accretion flow models. All power spectral densities of the hard-state X-ray light curves are describable as the sum of just a few broad, quasi-periodic features with frequencies that roughly scale as coronal compactness, ℓ<inf>c</inf>, to the −3/2 power. This is interpretable in a simple, toy model of an <it>efficient</it> spherical corona as variations of ℓ<inf>c</inf>∝<it>R</it><inf>t</inf>, where <it>R</it><inf>t</inf> is the ‘transition radius’ between the corona and an outer thin disc. Similar to observations of Cyg X-1, time lags between soft and hard variability anticorrelate with coronal compactness, and peak shortly after the transition from the soft to the hard state. A stronger correlation is seen between the time lags and the ‘reflection fraction’. These latter facts might suggest that the time lags are associated with the known, spatially very extended, synchrotron-emitting outflow.