Time evolution of the high-energy parameters of GX 339-4

The evolution of the high energy spectral tail is one of the open issues in the study of black-hole transients (BHT). We present broadband RXTE spectra for the 2007 outburst of GX 339-4, which show that the hard spectral component steepens throughout the overall transition from the hard to the soft state. The high-energy cutoff decreases monotonically during the brightening of the hard state, increases again during the softening in the hard intermediate state. It becomes unmeasurable in the soft states (possibly because of statistical limitations). This is the first time that the high-energy cutoff has been followed in such detail across several state transitions.