THE MAGNETIC-FIELD STRENGTH VERSUS ORBITAL PERIOD RELATION FOR BINARYRADIO PULSARS WITH LOW-MASS COMPANIONS - EVIDENCE FOR NEUTRON-STAR FORMATION BY ACCRETION-INDUCED COLLAPSE

The 24 binary radio pulsars with nearly circular orbits and low-mass c ompanions (0.2-0.4 M. helium white dwarfs in most cases) show a remark able correlation between spin period P and orbital period P-or, and be tween dipole surface magnetic field strength B-s and P-or. The observe d B-s vs. P-or relation is consistent with increasing decay of the neu tron-star magnetic field with increasing amounts of matter accreted, a s has been proposed on theoretical grounds by various authors. Neutron stars in binaries to which more than 0.45 f M. was transferred have f ield strengths below 10(9) G, where f(less than or equal to 1) is the fraction of the transferred matter that is not lost from the systems. The only one exception in the galactic disk (out of twelve systems) is the PSR 1831-00 system, in which greater than or equal to 0.7 f M. wa s transferred but the pulsar still has a strong magnetic held (similar to 0.8 x 10(11) G). Adopting the field decay by accretion models, the only way in which this can be explained is that the neutron star in t his system was formed near the end of the mass-transfer phase, by the accretion-induced collapse (AIC) of a white dwarf.