Constraints on accreting, isolated neutron stars from the Rosat and Euve surveys

Nearby old neutron stars can be detected by the radiation released from the accretion of interstellar material. We derive the expected numbers of such sources in current EUV/X-ray surveys, including the effects of dynamical heating on the speed distribution of the stellar population. Assuming blackbody emission and Ntot = 108N8 neutron stars in the Galaxy, we estimate that, in the case of isotropic accretion over the entire stellar surface and depending on the importance of dynamical heating, between 70 and 700N8 isolated neutron stars might have been detected in the ROSAT PSPC all-sky survey, with 4-50N8 of them showing up in the ROSAT WFC and EUVE surveys. An accretion flow channeled onto 1 km2 magnetic polar caps produces instead 500-4000N8 detectable X-ray sources in the PSPC survey, and only ≲4N8 EUV sources in the WFC and EUVE surveys. Such observations could constrain the accretion flow topology and the long-term evolution of isolated neutron stars, and we discuss potentially observable properties. The combined identification programs of the EUVE, WFC, and PSPC all-sky surveys might have already ruled out there being as many as 109 dead pulsars in the Galaxy.