Consider a pulsar that is near the high mass limit for a neutron star. It might acquire additional mass from the surroundings and become a rotating black hole. What happens to the magnetic field? Can the collapsed star still be a pulsar?
Can black holes be pulsars?
The "no hair" theorem says a black hole cannot maintain a magnetic field, it is a bit like a superconductor, the field lines are thought to be expelled from the event horizon according to present calculations. However, an accretion disc is a fast moving charged plasma so would almost certainly have a field. I think it therefore depends on whether the BH has a disc or has absorbed all the surrounding material.
Thanks George Dishman.
Your answer is helpful in some of my other threads.
Considering that the event horizon has no properties other than spin, gravity, and temperature I would guess that the gravity repels magnetic field lines. It agrees with Lagrangian methods, and with some of the metric solutions of GR.
Stephen Hawking discussed concepts of Magnetic black holes in chapter 3 of “The Nature of Space and Time,” 1994. Reference was made to Ernst 1976.
The result from decades of additional work seems to be that magnetic field lines don’t come out of or go into a black hole.
Short answer? No. There is more to being a pulsar than just having a rapid spin and strong magnetic field [and as noted already, no stellar magnetic field is believed to survive at the event horizon anyway].
Pulsar emission also requires a continual accelerated flux of baryonic particles - either pulled from the surface of the star and injected into the magnetosphere, in the case of a rotation-powered pulsar or field-powered magnetar; or impacting onto the magnetic poles of the star to create thermal hotspots, in the case of an accretion-powered pulsar.
Neither of these places is accessible in a BH.
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