The implications of radio-quiet neutron stars

We collate the evidence for rotation-powered neutron stars that are visible as X-ray sources and not as radio pulsars. Quantitative upper limits exist for the radio fluxes of eight objects, revealing a population at least an order of magnitude less luminous at radio wavelengths than known radio pulsars of similar power or age. These objects could have intrinsically low radio luminosities, but a simple alternative explanation is that they are pulsars in which the radio beams are directed away from Earth. Where hard X-ray or gamma-ray fluxes are seen, the beaming explanation implies different emission sites for the high-energy radiation and the unseen radio beams. We estimate that (a) the radio beaming fraction of young pulsars is roughly 50%, certainly much less than 100%, and (b) one neutron star is born in the Galaxy every ~90 years. We conclude that probably all neutron stars are born as radio pulsars and that most young, nearby pulsars have already been discovered.