THE ELUSIVENESS OF OLD NEUTRON-STARS

Old neutron stars (ONSs) that have radiated away their internal and ro tational energy may still shine if they are accreting the interstellar medium. Despite their large number, only two promising candidates hav e been detected so far, and rather stringent limits on their observabi lity follow from the analysis of ROSAT surveys. This contrasts with op timistic theoretical estimates that predicted a large number of source s in ROSAT fields. We have reconsidered the issue of the observability of ONSs, accounting for the spin and magnetic field evolution over th e neutron star lifetime. In the framework of a spin-induced held decay model, we show that the total number of ONSs that are at present in t he accretion stage is reduced by a factor of similar to 5 over previou s figures if the characteristic timescale for crustal current dissipat ion is similar to 10(8)-10(9) yr. This brings theoretical predictions much closer to observational limits. Most ONSs should be, at present, in the propeller phase; if subject to episodic flaring, they could be observable.