Spectral hardening as a viable alternative to disc truncation in black hole state transitions

Constraining the accretion flow geometry of black hole binaries in outburst is complicated by the inability of simplified multicolour disc models to distinguish between changes in the inner disc radius and alterations to the emergent spectrum, parametrized by the phenomenological colour-correction factor, f(col). We analyse Rossi X-ray Timing Explorer observations of the low-mass Galactic black hole X-ray binary, GX 339-4, taken over seven epochs when the source was experiencing a state transition. The accretion disc component is isolated using a pipeline resulting in robust detections for disc luminosities, 10(-3) less than or similar to L-disc/L-Edd less than or similar to 0.5. Assuming that the inner disc remains situated at the innermost stable circular orbit over the course of a state transition, we measure the relative degree of change in f(col) required to explain the spectral evolution of the disc component. A variable f(col) that increases by a factor of similar to 2.0-3.5 as the source transitions from the high/soft state to the low/hard state can adequately explain the observed disc spectral evolution. For the observations dominated by a disc component, the familiar scaling between the disc luminosity and effective temperature, L-disc proportional to T-eff(4), is observed; however, significant deviations from this relation appear when GX 339-4 is in the hard intermediate and low/hard states. Allowing for an evolving f(col) between spectral states, the L-disc proportional to T-eff(4) law is recovered over the full range of disc luminosities, although this depends heavily on the physically conceivable range of f(col). We demonstrate that physically reasonable changes in f(col) provide a viable description for multiple state transitions of a black hole binary without invoking radial motion of the inner accretion disc.