The observability of old neutron stars accreting the interstellar medium: III. The solar proximity

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Old isolated accreting neutron stars may show up among unidentified soft X--ray sources detected by the ROSAT All Sky Survey. We argue that the chances of identification are greater for ONSs located in the closest overdense regions of the solar neighbourhood. In particular, we consider the neutral hydrogen wall in the second Galactic quadrant (15∘ < l < 150∘) which shrinks the estimated contour of the Local Bubble to ≈ 16-30 pc. Due to their vicinity, we expect 10 ONSs to be detectable at a relatively high flux limit (0.1 counts/s) in the 0.2--2.4 keV band. This implies that about 5 % of unidentified sources above this threshold could be ONSs. No optical counterpart is present, but EUV emission from these objects could be detected using EUVE Lex filter at the highest sensitivity limits.