Spectral properties of transitions between soft and hard states in GX 339-4

We present a study of the spectral properties during the state transition of GX 339-4. Data are taken from the 2010 outburst of GX 339-4, which is densely covered by Rossi X-ray Timing Explorer, providing an excellent coverage of the state transitions between the low/hard state and the high/soft state. We select all observations within a certain hardness ratio range during the soft intermediate state (SIMS). This sample was chosen in such a way to comprise all observations that show a type-B quasi-periodic oscillation (QPO). In addition, we also investigate the spectra of hard intermediate state observations. The spectra, obtained from Proportional Counter Array data in the 1040 keV range, are fitted with a power law and an additional high-energy cut-off, if needed. We find that the spectra are significantly harder during the SIMS of the soft-to-hard transition than they are during the hard-to-soft transition. This demonstrates that during the SIMS of the soft-to-hard transition not only the luminosity and peak frequencies of type-B QPOs are lower, but also that the photon index is lower, compared to the hard-to-soft transition. Hence, type-B QPOs can be associated with a different spectral shape, even though they appear at the same hardness. However, in each branch only certain combinations of centroid frequency and photon index are realized.