Holger Stiele (Osservatorio Astronomico di Brera, Italy)

Spectral properties of the transition between soft and hard state of black hole binaries

Black hole X-ray binaries (BHBs) evolve through different states during an outburst. At the begin and end
BHBs are in the low hard state (LHS) – where X-ray emission is dominated by comptonised photons –
and in between they switch to the high soft state (HSS), where a thermal component is present. However the
exact properties and mechanism of the transitions are still under debate. In our study we relate spectral
characteristics to timing properties during state transitions. GX 339-4 is well suited for such a study,
especially the 2010 outburst, due to the excellent coverage of the state transitions between the LHS and
HSS with Rossi X-ray Timing Explorer (RXTE). We select all observations within a certain hardness ratio
range during the soft intermediate state (SIMS). This sample is 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. 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.