She's Got Her Mother's Hair: Unveiling the Origin of Black Hole Magnetic Fields through Stellar to Collapsar Simulations

TL;DR

This paper investigates the origin of magnetic fields in black holes powering gamma-ray bursts, proposing that progenitor neutron stars provide the magnetic flux, supported by stellar evolution and relativistic simulations.

Contribution

It introduces a novel scenario where black holes acquire magnetic flux from their progenitor neutron stars, challenging previous assumptions about core magnetic flux transport.

Findings

Progenitor core instabilities lead to low magnetic flux feeding the black hole.

Magnetized proto-neutron stars can supply magnetic flux for GRB jets.

Simulations confirm magnetic flux confinement and jet launching depend on black hole spin and disk presence.

Abstract

Relativistic jets from a Kerr black hole (BH) following the core collapse of a massive star ("collapsar") is a leading model for gamma-ray bursts (GRBs). However, the two key ingredients for a Blandford-Znajek 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 before collapse. Through innovative multidisciplinary modeling, we first use MESA stellar evolution models followed to core collapse, to 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 of Type Ic-BL supernovae, while the nascent BH's spin and magnetic flux produce jets consistent with observed GRB characteristics. 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.