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The accretion flow in the disk dominated state of black hole binaries haspeak temperature and luminosity which vary together in such a way as toindicate an approximately constant emitting area. The association of this withthe last stable orbit gives one of the few ways to estimate spin when the massof the black hole is known. However, deriving this radius requires knowledge ofhow the disk spectrum is modified by radiative transfer through the verticalstructure of the disk, as well as special and general relativistic effects onthe propagation of this radiation. Here we investigate the extent to whichdifferences in vertical structure change the derived disk spectra bycalculating these for a range of different stress prescriptions. We find thatat a given mass accretion rate the spectra are almost identical for accretionrates of L/L_Edd <~ 0.1. The spectra are remarkably similar even up to thehighest luminosities considered (L/L_Edd ~ 0.6) as long as the stresses do notdissipate more than about 10 per cent of the gravitational energy above theeffective photosphere. This is exceeded only by classic alpha disks with alpha>~ 0.1, but these models give spectral variation which is incompatible withexisting data. Therefore, we conclude that disk spectral modelling can placeinteresting constraints on angular momentum transport, but still provide arobust estimate of the spin of the black hole.

Angular Momentum Transport in Accretion Disks and its Implications for Spin Estimates in Black Hole Binaries