Are unidentified extreme-ultraviolet sources the closest neutron stars?

Unidentified extreme-ultraviotel (EUV) sources, detected in the EUVE and ROSAT WFC all-sky surveys, could be isolated old neutron stars, accreting material from the interstellar medium (ISM). The closest neutron stars, which are located in the local ISM bubble of unusually low density, are faint and cool ($$L\sim10^{27}\enspace\text{erg}\enspace\text s^{-1},\enspace T\lesssim 6 \enspace\text{eV}$$). The extreme UV spectrum of these sources is very sensitive to the H₁ column density, since a large fraction of the energy is emitted just below the hydrogen Lyman edge. The EUVE sources with large count rate in the long-wavelength bandpass (600 Å) seem to be the most promising candidates. These sources should have low H₁ column density ($$N_{\text H1}\lesssim10^{18}\enspace \text{cm}^{-2}$$), constraining their distance to a few tens of parsec. Otherwise, their spectra would be significantly modified by ISM absorption, and inevitably they would appear stronger in the short-wavelength (100 Å, 200 Å) EUVE bandpasses. If these unidentified objects are familiar EUV sources rather than neutron stars, i.e. white dwarfs, late type stars or cataclysmic variables, they are expected to be identifiable, and generally brighter than V ~14.