Do X-ray Binary Spectral State Transition Luminosities Vary?

Abstract

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 1$σ$ level. GRO J~1655-40, in particular, shows a state transition luminosity below the mean value for the other sources at the $4σ$ 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 $β=0.68$, with a most likely value of about $β=0.52$, and a minimum speed of $β=0.45$, 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 $\sim0.2c$. 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 $α$. 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.