Studying the extremely soft high-soft state of black hole X-ray binaries

During the 2009/2010 outburst of GX~339-4 and in the recent outburst of MAXI J1543-564, the black hole X-ray binaries went through a phase in their high/soft state in which their spectra were purely disc dominated. This means that no emission of a hot, Comptonized component was detected. We want to use XMM-Newton to study the spectral properties of such a disc dominated period in an outburst of either an already known or a new detected transient black hole X-ray binary with a high signal-to-noise spectrum. This proposal is aimed at studying in detail the very soft X-ray spectrum during this phase, including the behaviour of the iron line as well as the temporal variability of the soft X-ray emission. This observation will allow us to address the behaviour of the disc in this phase.