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

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