Consider a neutron star which is accruing matter from a companion. Eventually, it may pass the limit of stability and collapse into a black hole. Many, if not most, neutron stars have extremely intense magnetic fields which are "pinned" to the superconducting material of the star's core.

Such magnetic fields contain vast quantities of energy. Even the laboratory superconducting magnet fields have a volumetric energy density comparable to that of high explosive. Since the energy density goes as the square of the magnetic field strength, the vastly stronger fields of neutron stars possess volumetric energy contents comparable to that of a nuclear bomb, and they are much larger. The pressures are in the many trillions of atmospheres.

So long as the neutron star continues as such, these fields are stabilized by the superconducting interiors. However, once the even horizon forms, the situation changes radically. It is a theorem of General Relativity that black hole cannot have any magnetic field lines penetrating the event horizon. Therefore, at the instant of formation of the event horizon, all magnetic flux must be expelled from the incipient black hole.

The magnetic flux is thus decoupled from its former home, the late neutron star, and, since it is under stupendous pressure, expands at the speed of light. In the process, it must sweep all matter in its path before it, accelerating that matter to ultra-relativistic speeds. In addition, the rapid field change produces high energy gamma rays. This is, in my opinion, the source of the ultra-high energy cosmic rays and the gamma radiation bursts as well.

David F Mayer