Spectral properties of transitions between soft and hard state in GX
  339-4
  We present a study of the spectral properties during 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 10 to 40 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 that also the
photon index is lower, compared to the hard-to-soft transition. Hence, type-B
QPOs can be associated to 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 realised.
