The implications of radio-quiet neutron stars
K. T. S. Brazier (Physics Dept, University of Durham, UK),
S. Johnston (RCfTA, University of Sydney, Australia)
We collate the evidence for rotation-powered neutron stars that are visible as X-ray sources and not as radio pulsars. To date, ten objects have been proposed and one, Geminga, has been confirmed as a pulsar by the detection of 4.2 Hz pulsations. Several indicators have been used to support the proposition that the X-ray sources are isolated neutron stars, including high X-ray to optical/radio flux ratios, a constant X-ray flux and coincidence with a gamma-ray source. Seven of the published neutron star candidates are located near the centres of supernova remnants, two of them within plerions, suggesting that these are young objects (tau < 20,000 yr). The remaining candidate neutron stars have no associated supernova remnant and may be older systems, powered either by their rotation, like Geminga, or possibly by accretion from the interstellar medium.
Quantitative upper limits exist for the radio fluxes of eight of the ten objects and reveal a population at least an order of magnitude less luminous at radio wavelengths than known radio pulsars of similar power or age. These could be intrinsically low luminosity pulsars, but this implies an overpopulation of neutron stars relative to the galactic supernova rate. A simple alternative explanation within the context of existing pulsar models is that these objects are pulsars in which the radio beams are directed away from Earth. They are still visible as X-ray sources because the weakly modulated, surface (thermal) emission, which dominates the soft X-ray emission in most young to middle-aged radio pulsars, is radiated in all directions. In the cases where hard X-ray or gamma-ray fluxes are seen, the beaming explanation implies different emission sites for the non-thermal high-energy radiation and the unseen radio beams. From the number of candidate neutron stars and radio pulsars younger than 20,000 years and within 3.5 kpc, the radio beaming fraction of young pulsars is estimated to be roughly 50% and certainly much less than 100%. We find the local neutron star birth rate to be at least 13 Myr^-1 kpc^-2. This extrapolates to a galactic rate of one neutron star born 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.