On the Magnetic Field Evolution in Isolated Neutron Stars

We examine the discrepancy between the theoretically expected number of detectable isolated old neutron stars (10 2 -10 3 ), and the actually detected number of candidates (2-3). We argue that this discrepancy is explicable in terms of the suppression of accretion from the interstellar medium onto the old neutron stars. We show that for such a suppression to occur, a specific pattern for the magnetic field evolution in neutron stars is required. Strong magnetic fields ( B ≳ 10 12 G) are required to decay rapidly, while fields of the order of 10 10 G are required to remain constant in the absence of accretion. The required pattern agrees with recent theoretical models for magnetic field decay in neutron stars.