OLD ISOLATED ACCRETING NEUTRON STARS: CONTRIBUTION TO THE SOFT X--RAY BACKGROUND IN THE 0.5--2 KEV BAND

The issue of the observability of Old Isolated Neutron Stars (ONSs) accreting from the interstellar gas, is reconsidered using the spectra presented in Zampieri {\it et al.\/} (1995). In particular, we focus our attention on the overall soft X--ray emission of ONSs, which may provide a substantial contribution to the X--ray background in the 0.5--2 keV band. It is found that if accretion is channeled into polar caps by a magnetic field of $\sim 10^9$ G, up to $\sim$ $12-25 \%$ of the unresolved soft excess observed at high latitudes in the X--ray background can be explained in terms of emission from accreting isolated ONSs. However, this would imply the detection with ROSAT of $\sim 10$ sources deg$^{-2}$ above a threshold of $\sim 10^{-14} \, {\rm erg\, cm^{-2} s^{-1}}$. A brief reconsideration of the observability of isolated ONSs as individual X--ray sources is also presented.