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. 
K. T. S. Brazier (1) and S. Johnston (2) ((1) University of Durham,   (2) RCfTA, University of Sydney)