Effects of neutron-star superconductivity on magnetic monopoles and core field decay

From the magnetic properties of old neutron stars we propose that an observation of a sufficiently old pulsar limits any ``grand unified theory'' heavy magnetic monopole flux in the pulsar neighborhood to below 5×{}10^-24{{τ}}₁₀^-2{cm}^{{{-}}}$ $²sr^-1sec^-1, where τ₁₀ is the age (in 10¹⁰ yr) of the pulsar's present magnetic field and monopole speeds are {~}10^-3 c. For the millisecond pulsar PSR 1937+214 a major improvement over the Parker limit is obtained, which is also better than various limits from monopole catalysis of baryon decay, provided τ₁₀{}10^-1. The consideration of monopole dynamics inside superconducting neutron-star cores leads to this conclusion.