Do X-ray binary spectral state transition luminosities vary?

We tabulate the luminosities of the soft-to-hard state transitions of all X-ray binaries for which there exist good X-ray flux measurements at the time of the transition, good distance estimates, and good mass estimates for the compact star. We show that the state transition luminosities are at about 1 - 4% of the Eddington rate, markedly smaller than those typically quoted in the literature, with a mean value of 2%. Only the black hole candidate GRO J 1655-40 and the neutron star systems Aql X-1 and 4U 1728-34 have measured state transition luminosities inconsistent with this value at the 1sigma level. GRO J 1655-40, in particular, shows a state transition luminosity below the mean value for the other sources at the 4sigma level. This result, combined with the known inner disk inclination angle ( the disk is nearly parallel to the line of sight) from GRO J 1655-40' s relativistic jets suggest that the hard X-ray emitting region in GRO J 1655-40 can have a velocity of no more than about beta = 0.68, with a most likely value of about beta = 0.52, and a minimum speed of beta = 0.45, assuming that the variations in state transition luminosities are solely due to relativistic beaming effects. The variance in the state transition luminosities suggests an emission region with a velocity of similar to0.2c. The results are discussed in terms of different emission models for the low/hard state. We also discuss the implications for measuring the dimensionless viscosity parameter alpha. We also find that if its state transitions occur at typical luminosities, then GX 339-4 is likely to be at a distance of at least 7.6 kpc, much further than typically quoted estimates.