Isolated neutron stars: Accretors and coolers

As many as 10(9) neutron stars populate the Galaxy, but only approximate to 10(3) are directly observed as pulsars or as accreting sources in X-ray bi naries. In principle, also the accretion of the interstellar medium may mak e isolated neutron stars shine, and their weak luminosity could be detected in soft X-rays. Recent ROSAT observations have convincingly shown that neu tron stars accreting from the interstellar medium are extremely rare, if ob served at all, in contrast with earlier theoretical predictions. Until now two possible explanations for their elusiveness have been proposed: their v elocity distribution may peak at similar to 200-400 km s(-1), as inferred f rom pulsar statistics, and this would severely choke accretion; the magneti c field may decay on timescales similar to 10(8)-10(9) yr, preventing a lar ge fraction of neutron stars from entering the accretor stage. The search f or accreting neutron stars has produced up to now a handful of promising ca ndidates. While little doubt is left that these objects are indeed isolated neutron stars, the nature of their emission is still controversial. In par ticular, accreting objects can be confused with much younger, cooling neutr on stars. However, a combination of observations and theoretical modeling m ay help in discriminating between the two classes.