Activation of the Blandford-Znajek mechanism in collapsing stars

TL;DR

This paper investigates how the Blandford-Znajek mechanism can be activated during stellar collapse, potentially explaining gamma-ray bursts and hypernovae, by analyzing magnetic flux and rotation conditions near black holes.

Contribution

It demonstrates the activation conditions of the Blandford-Znajek mechanism in collapsing stars and discusses scenarios for magnetic field generation and their implications for stellar explosions.

Findings

BZ mechanism activates when magnetic energy density exceeds matter density near the black hole.

Strong magnetic fields can be sustained in binary systems, avoiding conflicts with stellar rotation.

Additional disk power can supplement energy output when magnetic flux is insufficient.

Abstract

Collapse of massive stars may result in formation of accreting black holes in their interior. The accreting stellar matter may advect substantial magnetic flux onto the black hole and promote release of its rotational energy via magnetic stresses (the Blandford-Znajek mechanism). In this paper we explore whether this process can explain the stellar explosions and relativistic jets associated with long Gamma-ray-bursts. In particularly, we show that the Blandford-Znajek mechanism is activated when the rest mass-energy density of matter drops below the energy density of magnetic field in the very vicinity of the black hole (within its ergosphere). We also discuss whether such a strong magnetic field is in conflict with the rapid rotation of stellar core required in the collapsar model and suggest that the conflict can be avoided if the progenitor star is a component of close binary. In this case the stellar rotation can be sustained via spin-orbital interaction. In an alternative scenario the magnetic field is generated in the accretion disk but in this case the magnetic flux through the black hole ergosphere is not expected to be sufficiently high to explain the energetics of hypernovae by the BZ mechanism alone. However, this energy deficit can be recovered via additional power provided by the disk.