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During the outbursts of black hole X-ray binaries (BHXRBs), their accretionflows transition through several states. The source luminosity rises in thehard state, dominated by non-thermal emission, before transitioning to theblackbody-dominated soft state. As the luminosity decreases, the sourcetransitions back into the hard state and fades to quiescence. This picture doesnot always hold, as $\approx$ 40$\%$ of the outbursts never leave the hardstate. Identifying the physics that govern state transitions remains one of theoutstanding open questions in black hole astrophysics. In this paper we presentan analysis of archival RXTE data of multiple outbursts of GX 339-4. We comparethe properties of the X-ray variability and time-averaged energy spectrum anddemonstrate that the variability (quantified by the power spectral hue)systematically evolves $\approx$ 10-40 days ahead of the canonical statetransition (quantified by a change in spectral hardness); no such evolution isfound in hard state only outbursts. This indicates that the X-ray variabilitycan be used to predict if and when the hard-to-soft state transition willoccur. Finally, we find a similar behavior in ten outbursts of four additionalBHXRBs with more sparse observational coverage. Based on these findings, wesuggest that state transitions in BHXRBs might be driven by a change in theturbulence in the outer regions of the disk, leading to a dramatic change invariability. This change is only seen in the spectrum days to weeks later, asthe fluctuations propagate inwards towards the corona.

Variability as a predictor for the hard-to-soft state transition in GX 339-4