Princeton University Astroplasmas Seminar

Jetted astrophysical phenomena with black hole (BH) engines, including binary mergers, jetted tidal disruption events, and X-ray binaries, require a large-scale vertical magnetic field for efficient jet formation.  I will present a possible mechanism for generating these crucial large-scale magnetic fields, using 3D global general relativistic magnetohydrodynamical (MHD) simulations of accretion disks. I find that the dynamo mechanism can be best understood as a nonlinear outcome of the magnetorotational instability (MRI) and large-scale advection. We characterize the complete dynamo mechanism with two timescales: one for local magnetic field generation and one for large-scale advection. The description and understanding of the dynamo mechanism pave the way toward a better understanding of jet launching. 

I will also describe the consequences of the large-scale vertical magnetic fields on the black hole. I will show that jet launching 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 black hole spin evolution model. 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.