The evolution of relativistic binary pulsars

Abstract

We address the question of why two of the four neutron star binaries, PSR 191316 and PSR 153412, have narrow orbits, with binary separation ~ 3 R ⊙, and relatively low magnetic fields B ~ 10 10G. Beginning from a scenario of double helium-star progenitors for binary pulsars, we show that the orbits will be tightened to ~ 3–5 yR ⊙ during a common envelope evolution during which the two helium stars expel the common hydrogen envelope. There is then, in the evolution, an “observability premium” if the helium-star, neutron-star intermediate stage is such that the binary is narrow in this stage. If so, accretion onto the neutron star from the helium star wind can lower its magnetic field, lengthening the pulsar spin-down time by doing so. The lower magnetic field B then has a longer spin-down time and, hence, a longer time of observability. Evolution of PSR 191316 from a binary of rather massive, main-sequence mass ~24 M ⊙ , O-stars proves to be straightforward, and the probability of such evolution in our model looks reasonable. We are unable to carry the lighter binary PSR 153412 through the second common envelope stage, reverse case BB mass transfer, which occurs when light-mass helium stars expand in a red supergiant phase, engulfing the neutron star. We suggest that, in some cases, the binary does not survive in this second common envelope, the neutron star either spiraling in to the helium star, or going into a black hole.