Background: BH accretion

There are three types of stable black hole disk solutions: advection-dominated accretion flow (ADAF), thin disk, and slim disk (see [1] and references therein for detailed review).

From low to high accretion rate, the accretion disk types are shown in Table 1. We can understand why the accretion disk should change its type by focusing on the cooling process of each type of accretion disk. Due to the release of large amount of gravitational energy, materials are hot enough to become plasmas. The two species of the plasma, ions and electrons, interact with each other by the Coulomb interactions.

When the dimensionless accretion rate is very low («1; corresponds to a ADAF), the interaction is so weak and hence ions cannot efficiently transfer their heat to electrons, the species which is much lighter and is mainly responsible for the radiation cooling. To maintain the local energy balance, it requires the advection flow to bring those heat inward (“advection cooling”). The heat stores inside the disk cause a large disk height.

When the dimensionless accretion rate is high enough (> 0.01) for the Coulomb interaction to be efficient at every radius of the disk , a radiation cooling disk is formed (corresponds to a thin disk). Such disk has a relatively low temperature compared to that of a ADAF. The disk height decreases and the disk become optically thick.

When the accretion rate further increases (≥1), the photon emitted from the electrons at the deeper layer of the disk are trapped by the disk itself (“photon-trapping”) and the advection of the flow play the major role in the cooling process again.

When the accretion rate increases from a small value and the disk transit form a ADAF (radiatively inefficient) to a thin disk (radiatively efficient), the accretion disk starts to cool down from the outer part of the disk. Therefore, before the entire accretion rate become a thin disk (which happen when the dimensionless accretion rate has a value ~0.01), a combined disk which consists of an outer thin disk and an inner ADAF, is formed. Different kind of accretion disk have different characteristic spectrum profile. As an example, different spectrum components of black hole X-ray binaries can be interpreted as contribution form the thin disk part (corresponding to the blackbody component of the spectra) and the ADAF part (corresponding to the power-law component of the spectra) of a combined disk [2].

[1] Kato, S., Fukue, J., & Mineshige, S. 2008, Black-Hole Accretion Disks — Towards a New Paradigm (Kyoto:KyotoUniv. Press)

[2] Remillard, R.A., & McClintock, J. E. 2006, ARA&A, 44,49