Modeling the Thick-to-thin Accretion Disk Transition in X-ray Binaries

Observations of X-ray binaries in the hard spectral state suggest that the thin accretion disk present far from the central black hole truncates to a thick, hot accretion flow at smaller distances. The location and structure of the thick-to-thin disk transition should depend on where Coulomb collisions become inefficient, allowing electrons and protons to decouple. However, studying the radial and vertical structure of this transition at intermediate mass accretion rates requires modeling radiation in a computationally inexpensive manner. To capture the impact of radiative cooling without evolving photon packets or a radiation fluid, we implement an electron-only cooling function in two-temperature general relativistic magnetohydrodynamic simulations. We systematically examine the thick-to-thin disk transition as a function of initial disk magnetization and mass accretion rate. This work will enable better understanding of X-ray binary state transitions.