Isolated Neutron Stars in the Galaxy

Abstract

In this article we briefly review our recent results on evolution and properties of isolated neutron stars (INSs) in the Galaxy. As the first step we calculate a {\it census} of INSs in our Galaxy. We infer a lower bound for the mean kick velocity of NSs, $<V>\sim$(200-300) $\mathrm{km}{\mathrm{s}}^{-1}$. The same conclusion is reached for both a constant magnetic field ($B\sim 10^{12}$ G) and for a magnetic field decaying exponentially with a timescale $\sim 10^9$ yr. These results, moreover, constrain the fraction of low velocity stars, which could have escaped pulsar statistics, to $\sim$few percents. Then we show that the range of minimum value of magnetic moment, ${\mu }_b$: $\sim 10^{29.5}\ge {\mu }_b\ge 10^{28}\mathrm{G}{\mathrm{cm}}^3$, and the characteristic decay time, $t_d$: $\sim 10^8\ge t_d\ge 10^7\mathrm{yrs}$, can be excluded assuming the standard initial magnetic momentum, ${\mu }_0=10^{30}\mathrm{G}{\mathrm{cm}}^3$, if accreting INSs are observed. For these parameters an INS would never reach the stage of accretion from the interstellar medium even for a low space velocity of the star and high density of the ambient plasma. The range of excluded parameters increases for lower values of ${\mu }_0$. It is shown that old accreting INSs become more abundant than young cooling INSs at X-ray fluxes below $\sim 10^{-13}$ erg cm${}^{-2}$ s${}^{-1}$. We can predict that about one accreting INS per square degree should be observed at the {\it Chandra} and {\it Newton} flux limits of $\sim 10^{-16}$ erg cm${}^{-2}$ s${}^{-1}.$ The weak {\it ROSAT} sources, associated with INSs, can be young cooling objects, if the NSs birth rate in the solar vicinity during the last $\sim 10^6$ yr was much higher than inferred from radiopulsar observations.

7 pages, 3 PostScript figures, to be presented at the conference "Hot points in astrophysics", Dubna, August 22-26, 2000