Spectral variability of GX 339-4 in a hard-to-soft state transition

We report on INTEGRAL observations of the bright black hole transient GX 339-4 performed during the period 2004 August-September, including the fast transition (10 h) observed simultaneously with INTEGRAL and RXTE on August 15 and previously reported. Our data cover three different spectral states, namely hard/intermediate state (HIMS), soft/intermediate state (SIMS) and high/soft state (HSS). We investigate the spectral variability of the source across the different spectral states. The hard X-ray spectrum becomes softer during the HIMS-to-SIMS transition, but it hardens when reaching the HSS. A principal component analysis demonstrates that most of the variability occurs through two independent modes: a pivoting of the spectrum around 6 keV (responsible for 75 per cent of the variance) and an intensity variation of the hard component (responsible for 21 per cent). The pivoting is interpreted as due to changes in the soft cooling photon flux entering the corona, the second mode as fluctuations of the heating rate in the corona. These results are very similar to those previously obtained for Cygnus X-1. Our spectral analysis of the spectra of GX 339-4 shows a high energy excess with respect to pure thermal Comptonization models in the HIMS: a non-thermal power-law component seems to be requested by data. In all spectral states joint IBIS, SPI and JEM-X data are well represented by hybrid thermal/non-thermal Comptonization (EQPAIR). These fits allow us to track the evolution of each spectral component during the spectral transition. The spectral evolution seems to be predominantly driven by a reduction of the ratio of the electron heating rate to the soft cooling photon flux in the corona, l(h)/l(s). The inferred accretion disc soft thermal emission increases by about two orders of magnitude, while the Comptonized luminosity decreases by at most a factor of 3. This confirms that the softening we observed is due to a major increase in the flux of soft cooling photons in the corona associated with a modest reduction of the electron heating rate.