By the no-hair theorem, black holes are only characterized by mass, charge and angular momentum. If the star is neutral, the black hole will have only mass and angular momentum – and therefore it cannot have a magnetic dipole field. Where did it go?
These three questions are similar but I think the answers will be different for each one:
What happens to an embedded magnetic field when a black hole is formed from rotating charged dust?
It seems to me a rotating charged black hole must have a dipole
magnetic field. But the strength of the dipole field seems like an
extra parameter that black holes are forbidden by the no-hair theorem.If a magnetic monopole falls into a schwarzchild black hole, what happens to the magnetic field?
Here there would be only radial magnetic field lines leaving from the
event horizon to infinity. So if magnetic charge is counted as charge
this should be no problem. But if the black hole were rotating
wouldn't that produce an electric dipole field?When a neutral star with a magnetic field collapses to form a black hole, what happens to the magnetic field?
Here there is no charge so how can there be a magnetic field
associated with a black hole? That would definitely violate the
no-hair theorem.
Best Answer
There is a test of the collapse of a magnetized neutron star (case 3) in this preprint: http://arxiv.org/abs/arXiv:1208.3487.
Here is some of the text and figures from that paper that describe how the magnetic field is expelled: