Can we observe accreting, isolated neutron stars?

We discuss the observability of isolated neutron stars (NSs) accreting interstellar material as sources of quiescent and transient UV and X-ray radiation. We study their spatial and kinematic properties in the solar neighborhood through Monte Carlo simulations of 105 orbits in the Galactic potential. We present a much faster semianalytic technique which is capable of reproducing the full kinematic properties of the local NS population, in remarkable agreement with the Monte Carlo results. We derive the accretion rate distributions associated with the various phases of the interstellar medium (ISM). Assuming blackbody emission and 109 NSs in the Galaxy, we estimate that, in the case of isotropic (polar cap) accretion, ∼2000 (10,000) old NSs should be observed as X-ray sources in the ROSAT XRT all-sky survey, with up to 600 (100) of them showing up at longer wavelengths in the ROSAT WFC survey. The number of detectable NSs in the forthcoming EUVE all-sky survey should be close to 200 (20). If old NSs are magnetized, we estimate that an additional 1000 sources located in giant molecular clouds should be observed in the ROSAT XRT survey. NSs detected by ROSAT XRT will be strongly concentrated toward the Galactic plane. Isolated NSs in the local cavity can contribute only 0.1% of the soft X-ray background at 100 eV. We argue, however, that the integrated emission from solitary NSs accreting material in the Galactic plane could give rise to the Galactic X-ray ridges observed by EXOSAT and HEAO 1 A-2. We investigate the emission properties of accreting NSs moving supersonically in dense atomic and molecular clouds, and show that they will produce elongated, "cometary" H II regions, a possible characteristic observational signature. Material accumulated by slow accretion onto the polar cap of magnetized NSs located in diffuse clouds might be unstable to nuclear burning and lead to X-ray bursts. We estimate a rate of 25 yr-1 energetic, ∼5 × 1037 ergs events within 1 kpc, which might be detectable by HETE. Such bursts may also be responsible for some of the fast X-ray transients observed by HEAO 1 A-l. We emphasize the sensitive dependence of these estimates on the uncertain kinematic properties of radio pulsars, total number of NSs produced throughout the history of the Galaxy, geography of the local ISM, magnetic field topology and evolution, physics of the accretion process, and emission properties.