Evidence for an asymptotic lower limit to the surface dipole magnetic field strengths of neutron stars

The discovery of a second millisecond binary radio pulsar¹, PSR1855+09 (period P = 5ms), with a relatively wide circular orbit and low mass function indicates that the incidence of such systems (see also refs 2, 3) among the galactic radio pulsar population is ∼4 × ≳10^-3. The generally accepted model for the origin of these systems is that they have descended from low-mass X-ray binaries in which a weakly magnetized neutron star³ was spun-up by the accretion of matter from an evolved low-mass (≲1.2 M_⊙) companion star, which has now ended its life as a helium white dwarf^3-8. We show here that, based on the incidence of the progenitor low-mass X-ray binaries (LMXBs), the observed incidence of millisecond binary radio pulsars can be explained only if, at their present low field strength of ∼10⁹G, the decay timescale of neutron-star surface magnetic dipole moments is 322153a010⁹ yr. This possibility has been advanced by Kulkarni⁹, who estimates, based on the optical detection of the counterpart of the weak-field binary radio pulsar PSR0655 + 64, that this pulsar is old (>5 × 10⁸ yr) and hence that the magnetic field decay timescale becomes very large (>10⁹ yr) for field strengths ≲10¹⁰ G. Our findings provide independent support for this possibility.