Large-scale magnetic-field generation in turbulent black-hole accretion disks and its imprint on black hole spin.

Binary mergers involving black hole engines require a substantial large-scale vertical magnetic field for jet propulsion. I will present a possible mechanism for generating these crucial magnetic fields in binary mergers, using 3D global general relativistic magnetohydrodynamical (MHD) simulations of accretion disks. My investigation unveils that the dynamo mechanism can be best understood as a nonlinear outcome of the magnetorotational instability (MRI). We characterize the complete dynamo mechanism with two timescales: one for the local magnetic field generation and one for the large-scale scale advection. The description and understanding of the dynamo mechanism pave the way toward a better understanding of jet launching in binary mergers. My results are also salient for other astrophysical objects, like, for example, X-ray binaries and Tidal disruption events. I will also describe the consequences of the large-scale vertical magnetic fields on the black hole. I will show that jet propulsion through a large-scale field leads to efficient spin-down of the central black hole, reducing the dimensionless black hole spin from a=1 to a=0.2 after accreting only 20% of its initial mass. By separating the contributions of the accretion disk and the large-scale magnetic field, we constructed a simplified model for black hole spin evolution. I used this model to explore the consequence of black hole spin-down in the context of collapsing stars. My results show that collapsar black holes are born slowly spinning and should remain slowly spinning, consistent with LIGO/VIRGO/KARGA constraints on black hole spin.