On the number of accreting and cooling isolated neutron stars detectable with the ROSAT All-Sky Survey

We present limits to the log N - log S curve for isolated neutron stars, both cooling and accreting neutron stars, which are not active as radio pulsars, as observed with the ROSAT All-Sky Survey and compare it with theoretical expectations. So far, only one isolated neutron star is identified optically among ROSAT sources, namely RXJ185635-3754 (Walter & Matthews 1997). Three more promising candidates have been suggested. In addition, several upper limit estimates are available on the space density of such neutron stars from different optical follow-up studies. We show that the log N - log S curve according to the current observations, including the identified neutron star, the three additional candidates, and the upper limits, lies between the theoretical expectations for middle-aged cooling neutron stars and old accreting neutron stars. At least one of the neutron star candidates found so far with ROSAT may be cooling instead of accreting. We suggest that the fact that more accreting isolated old neutron stars were expected (e.g., Madau & Blaes 1994) than observed is mostly due to the velocity distribution used in those calculations. More recent radio observations indicate that there are fewer slow neutron stars, ie., fewer accreting X-ray bright old neutron stars. At the X-ray bright end of the log N - log S curve, however, the ROSAT observations agree well with the theoretical expectations.