Accretion induced black hole spin up revised by numerical GR MHD simulations

We investigate the accretion induced spin up of the black hole via numerical simulations. Our method is based on general-relativistic magneto-hydrodynamics of the slowly-rotating flows in the Kerr metric, where possibly transonic shock fronts may form. We account for the changing black hole mass and spin during accretion which enforces dynamical evolution of the space-time metric. We first study non-magnetized flows with shocks, and we also include magnetic field endowed in the gas. The aim of this study is to verify whether the high mass black holes may be produced with large spins, even though at birth the collapsars might have contained slowly, or moderately spinning cores. In this way, we put constraints on the content of angular momentum in the collapsing massive stars. Our studies are also showing that shock fronts and magnetic fields may halt accretion and limit the black hole spin-up in the exploding supernovae.