THE OBSERVABILITY OF OLD ISOLATED NEUTRON-STARS WITH ROSAT .2. MOLECULAR CLOUDS AND DEEP FIELDS

The possibility of detecting with ROSAT old isolated neutron stars (ONSs) accreting the interstellar medium is re-examined. Their observability in molecular clouds is firstly discussed. Considering in particular the inventory of clouds by Dame et al. (1987), it is found that a few ONSs detectable above the ROSAT All Sky Survey limit should be present in most of the clouds, and in some case they should be numerous. ROSAT deep exposures are then considered and it is shown that in the 10 square degrees explored thus far (Hasinger 1992) the number of detectable ONSs should be at most approximately 5. In this case the perspective of distinguishing ONSs from other sources seems rather arduous.