Hair of astrophysical black holes

TL;DR

This paper demonstrates that rotating neutron stars collapsing into black holes can retain magnetic flux, challenging the traditional 'no hair' theorem by showing long-term magnetic field retention due to plasma effects.

Contribution

It introduces a topological constraint preventing magnetic flux from dissipating during collapse, leading to long-lived magnetic fields around black holes formed from neutron stars.

Findings

Black holes formed from neutron stars can retain magnetic flux for long times.

The magnetosphere relaxes to a split monopole configuration after collapse.

Flux tubes are conserved during collapse, contrary to classical expectations.

Abstract

The "no hair" theorem is not applicable to black holes formed from collapse of a rotating neutron star. Rotating neutron stars can self-produce particles via vacuum breakdown forming a highly conducting plasma magnetosphere such that magnetic field lines are effectively "frozen-in" the star both before and during collapse. In the limit of no resistivity, this introduces a topological constraint which prohibits the magnetic field from sliding off the newly-formed event horizon. As a result, during collapse of a neutron star into a black hole, the latter conserves the number of magnetic flux tubes N_B = e \Phi_\infty /(\pi c \hbar), where \Phi_\infty is the initial magnetic flux through the hemispheres of the progenitor and out to infinity. The black hole's magnetosphere subsequently relaxes to the split monopole magnetic field geometry with self-generated currents outside the event horizon. The dissipation of the resulting equatorial current sheet leads to a slow loss of the anchored flux tubes, a process that balds the black hole on long resistive time scales rather than the short light-crossing time scales expected from the vacuum "no-hair" theorem.