She’s Got Her Mother’s Hair: End-to-End Collapsar Simulations Unveil the Origin of Black Holes' Magnetic Field

She’s Got Her Mother’s Hair: End-to-End Collapsar Simulations Unveil the Origin of Black Hole's Magnetic Field (2024)

The formation of a relativistic jet from a Kerr black hole (BH) following the core-collapse of a massive star ("collapsar") is a leading model for gamma-ray bursts (GRBs), the most luminous electromagnetic transients in the universe. However, the two key ingredients for a Blandford-Znajek (BZ) powered jet - rapid rotation and a strong magnetic field - seem mutually exclusive. Strong fields in the progenitor star's core transport angular momentum outwards more quickly, slowing down the core prior to collapse. Using MESA stellar evolution models followed to core-collapse, we explicitly show that the small length-scale of the instabilities, likely responsible for angular momentum transport in the core (e.g., Tayler-Spruit), results in a low net magnetic flux fed to the BH horizon, far too small to power GRB jets. Instead, we propose a novel scenario in which collapsar BHs acquire their magnetic ``hair'' from their progenitor proto-neutron star (PNS), which is likely highly magnetized from an internal dynamo. We evaluate the conditions for the BH accretion disk to pin the PNS magnetosphere to its horizon immediately after the collapse. Our results show that the PNS spin-down energy released before collapse matches the kinetic energy in Ic-BL supernovae, while the nascent BH's spin and magnetic flux produce jets consistent with the characteristics observed in GRBs. We map our MESA models to 3D general-relativistic magnetohydrodynamic simulations and confirm that accretion disks confine the strong magnetic flux initiated near a rotating BH, enabling the launch of successful GRB jets, whereas a slower spinning BH or one without a disk fails to do so.