Magnetized Accretion Flows: Effects of Gas Pressure
  We study how axisymmetric magnetohydrodynamical (MHD) accretion flows depend
on gamma adiabatic index in the polytropic equation of state. This work is an
extension of Moscibrodzka & Proga (2008), where we investigated the gamma
dependence of 2-D Bondi-like accretion flows in the hydrodynamical (HD) limit.
Our main goal is to study if simulations for various gamma can give us insights
into to the problem of various modes of accretion observed in several types of
accretion systems such as black hole binaries (BHB), active galactic nuclei
(AGN), and gamma-ray bursts (GRBs). We find that for gamma >~ 4/3, the fast
rotating flow forms a thick torus that is supported by rotation and gas
pressure. As shown before for gamma=5/3, such a torus produces a strong,
persistent bipolar outflow that can significantly reduce the polar funnel
accretion of a slowly rotating flow. For low gamma, close to 1, the torus is
thin and is supported by rotation. The thin torus produces an unsteady outflow
which is too weak to propagate throughout the polar funnel inflow. Compared to
their HD counterparts, the MHD simulations show that the magnetized torus can
produce an outflow and does not exhibit regular oscillations. Generally, our
simulations demonstrate how the torus thickness affects the outflow production.
They also support the notion that the geometrical thickness of the torus
correlates with the power of the torus outflow. Our results, applied to
observations, suggest that the torus ability to radiatively cool and become
thin can correspond to a suppression of a jet as observed in the BHB during a
transition from a hard/low to soft/high spectral state and a transition from a
quiescent to hard/low state in AGN.
