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

Abstract

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, fcol.We analyse Rossi X-ray Timing Explorer observations of the lowmass 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 (≲ Ldisc/LEdd ≲ 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 fcol required to explain the spectral evolution of the disc component. A variable fcol that increases by a factor of ~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, Ldisc α T4eff, 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 fcol between spectral states, the Ldisc-T4eff law is recovered over the full range of disc luminosities, although this depends heavily on the physically conceivable range of fcol.We demonstrate that physically reasonable changes in fcol provide a viable description for multiple state transitions of a black hole binary without invoking radial motion of the inner accretion disc. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.