Constraining the accretion flow geometry of black hole binaries in outburst is complicated by the inability of simplified multi-colour disc models to distinguish between changes in the inner disc radius and alterations to the emergent spectrum , parameterised 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 } \lesssim L _ { disc } / L _ { Edd } \lesssim 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 \sim 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 } \propto 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 } - 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 .