ARE THERE ANY ISOLATED OLD NEUTRON-STARS IN THE ROSAT WIDE-FIELD CAMERA SURVEY

We present the results of Monte Carlo simulations to predict the numbe r of isolated old neutron stars (IONs) that are observed in the ROSAT Wide Field Camera (WFC) survey as a result of accretion-powered extrem e ultraviolet (EUV) emission. Magnetic field strengths of 10(9) to 10( 12) G are considered along with different models for the local interst ellar medium. The most recent calibrations of the instrumental respons e and spatial sensitivity variations are used. Our conclusions are tha t as many as 10-20 IONs may have been observed if initial magnetic fie lds decay quickly to values less than or equal to 10(10) G, but higher fields result in far fewer detections. On the basis of the latest sou rce lists and optical identification information, which essentially ru le out the presence of more than one or two IONs in the WFC survey, we conclude that one or all of the following is possible. (i) The number of IONs in the Galaxy is less than 10(9) or the number of accreting I ONs may be depleted by accretion-prevention mechanisms. The latter is considered unlikely because the low-speed IONs that are most likely to have overcome the accretion barriers form the majority of the detecte d EUV sources. (ii) ION magnetic fields have not decayed to values les s than or equal to 10(10) G. (iii) The number of low-speed IONs may be fewer than expected, because of either dynamical heating or uncertain ties in the birth distribution of pulsar speeds. (iv) The ION emission spectrum may be significantly non-Planckian, with a reduced flux in t he WFC band.