The evolution of the high-energy cut-off in the X-ray spectrum of GX 339-4 across a hard-to-soft transition

We report on X-ray observations of the black hole candidate GX 339-4 during its 2006/2007 outburst. The hardness-intensity diagram (HID) of all RXTE/Proportional Counter Array data combined shows a q-shaped track similar to that observed in previous outbursts. The evolution through the HID suggests that in the early phase of the outburst the source underwent a sequence of state transitions, from the hard to the soft state, which is supported by our timing analysis. Broad-band (4-200keV) spectra, fitted with an exponentially cut-off power law, show that the hard spectral component steepens during the transition from the hard to the soft state. The high-energy cut-off decreased monotonically from 120 to 60keV during the brightening of the hard state, but again increased to 100keV during the softening in the hard intermediate state. In the short-lived soft intermediate state the cut-off energy was ~130keV, but was no longer detected in the soft state. This is one of the first times that the high-energy cut-off has been followed in such detail across several state transitions. We find that in comparison to several other spectral parameters, the cut-off energy changes more rapidly, just like the timing properties. The observed behaviour of the high-energy cut-off of GX 339-4 is also similar to that observed with RXTE-INTEGRAL-Swift during the 2005 outburst of GRO J1655-40. These results constitute a valuable reference to be considered when testing theoretical models for the production of the hard component in these systems.