Old isolated neutron stars in the galaxy: Evolution and field decay

Abstract

Abstract Old isolated neutron stars form a large population of the Galactic objects, and at the present time most of them are unobserved. In this talk we briefly describe their main properties and show some examples. As an example we discuss possible evolution of the spin period and the magnetic field of the X-ray source RX J0720.4-3125 assuming this source to be an isolated neutron star accreting interstellar medium. Magnetic field of the source is estimated to be 106-109G, and it is difficult to explain observed spin period 8.39 s without invoking hypothesis of the magnetic field decay. We used the model of ohmic decay of the crustal magnetic field. The estimates of accretion rate (10−14—10−16M[odot]/yr), velocity of the source relative to interstellar medium (10–50km/s), neutron star age (2.109-1010yrs) are obtained.