GX 339—4: the distance, state transitions, hysteresis and spectral correlations

We study X-ray and variability and distance of GX 339–4. We derive the distance of > 7 kpc, based on recent determination of the binary parameters. We study data from the ASM aboard Ginga, the BATSE aboard CGRO, and the ASM, PCA and HEXTE aboard RXTE. From 1987 to 2004, GX 339–4 underwent �15 outbursts and went through all known states of black-hole binaries. For the first time, we present the PCA data from the initial hard state of the outburst of 2004. We then study colourcolour and colour-flux correlations. In the hard state, there is a strong anticorrelation between the 1.5–5 and 3–12 keV spectral slopes, which we explain by thermal Comptonization of disc photons. There is also a softening of the spectrum above 3 keV with the increasing flux that becomes stronger with increasing energy up to �200 keV. This indicates an anticorrelation between the electron temperature and luminosity, explained by hot accretion models. In addition, we see a variable broad-band slope with a pivot at �200 keV. In the soft state, there is a high energy tail with varying amplitude beyond a strong and variable blackbody component. We confirm the presence of pronounced hysteresis, with the hard-to-soft state transitions occurring at much higher (and variable) luminosities than the soft-to-hard transitions. We fit the RXTE/ASM data with a model consisting of an outer accretion disc and a hot inner flow. State transitions are associated then with variations in the disc truncation radius, which we fit as � 6GM/c 2 in the soft state and several times that in the hard state. The disappearence of the inner disc takes place at a lower accretion rate than its initial appearance due to the dependence of the transitions on the source history. We provide further evidence against the X-ray emission in the hard state being nonthermal synchrotron, and explain the observed radio-X-ray correlation by the jet power being correlated with the accretion power.