2002ApJ...581..431M

We study the structure and properties of hot MHD accretion onto a Kerr black hole (BH). In such a system, the hole is magnetically coupled to the inflowing gas and exerts a torque onto the accretion flow. A hot settling flow can form around the hole and transport the angular momentum outward, to the outer edge of the flow. Unlike other hot flows, such as advection- and convection-dominated flows and inflow-outflow solutions (ADAFs, CDAFs, and ADIOS), the properties of the hot settling flow are determined by the spin of the central BH but are insensitive to the mass accretion rate. Therefore, it may be possible to identify rapidly spinning BHs simply from their broadband spectra. Observationally, the hot settling flow around a Kerr black hole is somewhat similar to other hot flows in that they all have hard, power-law spectra and relatively low luminosities. Thus, most BH candidates in the low/hard and, perhaps, intermediate X-ray state may potentially accrete via the hot settling flow. However, a settling flow will be somewhat more luminous than ADAFs/CDAFs/ADIOS, will exhibit high variability in X-rays, and may have relativistic jets. This suggests that Galactic microquasars and active galactic nuclei may be powered by hot settling flows. We identify several Galactic X-ray sources as the best candidates.