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. Inprinciple also the accretion of the interstellar medium may make isolatedneutron stars shine, and their weak luminosity could be detected in softX-rays. Recent ROSAT observations have convincingly shown that neutron starsaccreting from the interstellar medium are extremely rare, if observed at all,in contrast with earlier theoretical predictions. Until now two possibleexplanations for their elusiveness have been proposed: their velocitydistribution may peak at $\sim 200-400 {\rm km s}^{-1}$, as inferred frompulsar statistics, and this would severely choke accretion; the magnetic fieldmay decay on timescales $\sim 10^8-10^9$ yr, preventing a large fraction ofneutron stars from entering the accretor stage. The search for accretingneutron stars has produced up to now a handful of promising candidates. Whilelittle doubt is left that these objects are indeed isolated neutron stars, thenature of their emission is still controversial. In particular accretingobjects can be confused with much younger, cooling neutron stars. However, acombination of observations and theoretical modeling may help in discriminatingbetween the two classes.Comment: 22 pages Latex, 3 Figures, Invited Review to be published to PAS