Are Unidentified EUV Sources the Closest Neutron Stars?

Abstract

Unidentified extreme UV sources, detected in the {\it EUVE} and {\it ROSAT} WFC all-sky surveys, could be isolated old neutron stars, accreting material from the ISM. The closest neutron stars, that are located in the local ISM bubble of unusually low density, are faint and cool ($L \sim 10^{27} {\rm \ erg \ s^{-1}}$, $T \sles 6 $eV). The extreme UV spectrum of these sources is very sensitive to the H I column density, since large fraction of the emission is emitted just below the hydrogen Lyman edge. The {\it EUVE} sources with large count rate in the long-wave bandpass (600\AA) seem to be the most promising candidates. These sources should have low H I column density ($N_{HI} \sles 10^{18} {\rm \ cm^{-2}}$), constraining their distance to a few tens of parsec. Otherwise, their spectra would be significantly modified by ISM absorption, inevitably become stronger in the short wavelength (100\AA, 200\AA) {\it EUVE} bandpasses. If these unidentified objects are familiar EUV sources rather than neutron stars, either white dwarfs, late type stars or cataclysmic variables, they are expected to be identifiable, generally brighter than $\sim 14$.