Coronal–Temporal Correlations in GX 339−4: Hysteresis, Possible Reflection Changes, and Implications for ADAFs

We present spectral fits and timing analysis of Rossi X-ray Timing Explorer 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 Advanced Satellite for Cosmology and Astrophysics observations. Hysteresis in the soft state/hard state transition is observed. The hard state exhibits a possible anti-correlation 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 (PSD) of the hard state X-ray lightcurves are describable as the sum of just a few broad, quasi-periodic features with frequencies that roughly scale as coronal compactness, lc, to the −3/2 power. This is interpretable in a simple, toy model of an efficient spherical corona as variations of lc ∝ Rt, where Rt 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 anti-correlate with coronal compactness, and peak shortly after the transition from the soft to 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.