2007ApJ...657..428W

We argue, following our earlier works (the ``CEBZMC model''), that the phenomenon of twin-peak high-frequency quasi-periodic oscillations (QPOs) observed in black hole X-ray binaries is caused by magnetic coupling (MC) between the accretion disk and the black hole (BH). Due to MC, two bright spots occur at separate radial locations r<sub>in</sub>and r<sub>out</sub>at the disk surface, energized by a kind of Blandford-Znajek mechanism (BZ). We assume, following the Kluzniak-Abramowicz QPO resonance model, that the Keplerian frequencies at these two locations are in the 3:2 ratio. With this assumption, we estimate the BH spins in several sources, including GRO J1655-40, GRS 1915+105, XTE J1550-564, H1743-322, and Sgr A*. We give an interpretation of the ``jet line'' in the hardness-intensity plane, discussing the parameter space consisting of the BH spin and the power-law index for the variation of the large-scale magnetic field in the disk. Furthermore, we propose a new scenario for the spectral state transitions in BH X-ray binaries based on fluctuations in the densities of the accreting plasma from a companion star.