Do X-ray binary spectral state transition luminosities vary?

Astrophysics Sector, Scuola Internazionale Superiore di Studi Avanzati, via Beirut, n. 2-4, 34014 Trieste, Italy and Astronomical Institute “Anton Pannekoek,” University of Amsterdam, Kruislaan 403,1098 SJ Amsterdam, The Netherlands

Corresponding author: tjm@science.uva.nl

Received: 11 June 2003
Accepted: 18 July 2003

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 1σ level. GRO J 1655-40, in particular, shows a state transition luminosity below the mean value for the other sources at the 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 , with a most likely value of about , and a minimum speed of , 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 ~0.2c. 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.