  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.

High Energy Astrophysical Phenomena