Isolated Neutron Stars: Accretors and Coolers

As many as $10^9$ neutron stars populate the Galaxy, but only $\approx 10^3$ are directly observed as pulsars or as accreting sources in X-ray binaries. In principle also the accretion of the interstellar medium may make isolated neutron stars shine, and their weak luminosity could be detected in soft X-rays. Recent ROSAT observations have convincingly shown that neutron stars accreting from the interstellar medium are extremely rare, if observed at all, in contrast with earlier theoretical predictions. Until now two possible explanations for their elusiveness have been proposed: their velocity distribution may peak at $\sim 200-400 {\rm km s}^{-1}$, as inferred from pulsar statistics, and this would severely choke accretion; the magnetic field may decay on timescales $\sim 10^8-10^9$ yr, preventing a large fraction of neutron stars from entering the accretor stage. The search for accreting neutron stars has produced up to now a handful of promising candidates. While little doubt is left that these objects are indeed isolated neutron stars, the nature of their emission is still controversial. In particular accreting objects can be confused with much younger, cooling neutron stars. However, a combination of observations and theoretical modeling may help in discriminating between the two classes.