Binary pulsar J0737−3039 – evidence for a new core collapse and neutron star formation mechanism

The binary pulsar J0737−3039 is the only known system having two observable pulsars, thus offering a unique laboratory to test general relativity and explore pulsar physics. Based on the low eccentricity and the position within the galactic plane, two of us have argued that pulsar B had a non-standard formation scenario with little or no mass ejection and predicted that the system would have a very slow proper motion. Pulsar timing measurements confirmed this prediction. The recent observations of the alignment between the spin of pulsar A and the binary orbit is also in agreement with this scenario. Detailed simulations of the formation process of pulsar B show that its progenitor, just before the collapse, was a massive O–Ne–Mg white dwarf surrounded by a tenuous, 0.1–0.16 M⊙, envelope. This envelope was ejected when the white dwarf collapsed to form a neutron star. Pulsar B was born as a slow rotator (spin period ∼1 s) and a kick received when the pulsar formed changed its spin direction to the current one. This realization sheds light on the angular momentum evolution of the progenitor star, a process which is strongly affected by interaction with the binary companion. The slow proper motion of the system also implies that the system must have undergone a phase of mass transfer in which star A shed a significant fraction of its mass on to B.