Abstract

We investigate the role of radiative cooling in determining the geometric structure of accretion flows around black holes (BHs), and develop a model to explain the time evolution of observed transients in BH X-ray binaries (XRBs). Based on our numerical simulations, which include bremsstrahlung cooling, we arrive at the condition that the cooling time of the flow should be shorter than the viscous time, in order for a transition to occur from of a hot, geometrically thick, radiatively inefficient accretion flow (RIAF), to a cold, geometrically thin disk. We establish that variability in mass accretion rate ( Ṁ ), together with viscous evolution, can explain BH state transitions. Interestingly, our model qualitatively reproduces the overall features of the observed state transitions in XRBs.