How does the mass accretion rate affect these transitions? Further can the hardening or softening of the spectrum in the accretion be related to the changes in the shock properties in accretion flows, the shock strength and compression ratio for instance?
Thanks Joseph, but I think your answer is out of context. I am talking about state transitions of the black hole accretion disk spectra. What is the most promising model for explaining the low/hard, quiescent and high/soft spectra in the accretion disks?
This is still very much an open question.
Recent ideas include:
Turbulence in a magnetic field -
Begelman, M. \& Armitage, P. 2014
arXiv:1401.5475
The tearing off of rings from the accretion disk -
Nixon, C. \& Salvesen G. 2014,
MNRAS 437,3994
An older idea is that of magnetically arrested disks (MAD). Here, material initially builds up against a magnetic barrier while in the hard state. When it breaks through, significant mass accumulates close to the black hole, heats up and emits a soft spectrum which outshines the harder power-law component.
e.g. Narayan, Igumenschev & Abramowicz PASJ: Publ. Astron. Soc. Japan 55, L69-L72, 2003
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