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 (4–200 keV) 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 60 keV during the brightening of the hard state, but again increased to 100 keV during the softening in the hard intermediate state. In the short-lived soft intermediate state the cut-off energy was ∼130 keV, 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.