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Stellar-mass black hole binary systems in the luminous X-ray states show astrong quasi-periodic oscillation (QPO) in their Comptonised emission. Thefrequency of this feature correlates with the ratio of a disc to Comptonisedemission rather than with total luminosity. Hence it changes dramaticallyduring spectral transitions between the hard and soft states. Its amplitude isalso strongest in these intermediate states, making them an important test ofQPO models. However, these have complex spectra which generally require a discand two separate Comptonisation components, making it difficult to uniquelyderive the spectral parameters. We build a new energy-conserving model of theaccretion flow, SSsed model, which assumes a fixed radial emissivity but with achanging emission mechanism. This is similar to the agnsed model in xspec buttuned to be more suitable for stellar mass black holes. It uses a combinationof the disc luminosity and temperature to constrain the inner radius of the(colour temperature corrected) blackbody disc, separating this from the morecomplex Comptonisation spectra emitted inwards of this radius. We show a pilotstudy of this model fit to hundreds of RXTE spectra of the black hole binaryXTE J1550-564. We show that the derived disc radius tightly anti-correlateswith the central frequencies of the low-frequency QPO detected in the sameobservations. The relation is consistent with the quantitative predictions ofLense-Thirring precession of the entire inner Comptonisation regions for theassumed system parameters. This supports the scenario that low-frequency QPOsare caused by Lense-Thirring precession.

Disc corona radii and QPO frequencies in black hole binaries: testing Lense-Thirring precession origin