NEUTRON-STARS ACCRETING THE ISM - ARE THEY FAST OR SLOW OBJECTS

Old neutron stars (ONSs) which have radiated away their internal and r otational energy may still shine if accreting the interstellar medium. Rather stringent Limits from the analysis of ROSAT surveys indicate t hat most optimistic predictions on ONSs observability are in excess of a factor as large as similar to 100. Here we explore two possible evo lutionary scenarios that may account for the paucity of ONSs. In the f irst it is assumed that the ONS population is not too fast (V < 100 km s(-1)) and that magnetic field decay guides the evolution. In the sec ond, NSs move with high speed (V > 100 km s(-1)) and preserve their ma gnetic field at birth. We find that according to the former scenario m ost ONSs are now in the propeller phase, while in the latter nearly al l ONSs are silent, dead pulsars.