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Radiatively Inefficient Accretion Flow Simulations with Cooling: Implications for Black Hole Transients

We study the effects of optically thin radiative cooling on the structure of radiatively inefficient accretion flows (RIAFs). The flow structure is geometrically thick, and independent of the gas density and cooling, if the cooling time is longer than the viscous timescale (i.e., $t_{\rm cool} \gtrsim t_{\rm visc}$). For higher densities, the gas can cool before it can accrete and forms the standard geometrically thin, optically thick Shakura-Sunyaev disk. For usual cooling processes (such as bremsstrahlung), we expect an inner hot flow and an outer thin disk. For a short cooling time the accretion flow separates into two phases: a radiatively inefficient hot coronal phase and a cold thin disk. We argue that there is an upper limit on the density of the hot corona corresponding to a critical value of $t_{\rm cool}/t_{\rm ff} (\sim 10-100)$, the ratio of the cooling time and the free-fall time. Based on our simulations, we have developed a model for transients observed in black hole X-ray binaries (XRBs). An XRB in a quiescent hot RIAF state can transition to a cold black-body dominated state because of an increase in the mass accretion rate. The transition from a thin disk to a RIAF happens because of mass exhaustion due to accretion; the transition happens when the cooling time becomes longer than the viscous time at inner radii. Since the viscous timescale for a geometrically thin disk is quite long, the high-soft state is expected to be long-lived. The different timescales in black hole transients correspond to different physical processes such as viscous evolution, cooling, and free-fall. Our model captures the overall features of observed state transitions in XRBs.