Origin of the Binary Pulsar J0737-3039B

Evolutionary scenarios suggest that the progenitor of the new binary pulsar J0737-3039B was a He star with $M>(2.1–2.3){\mathrm{M}}_{\odot }$. We show that this case implies that the binary must have a large ($>120\mathrm{k}\mathrm{m}/\mathrm{s}$) center of mass velocity. However, the location, $\sim 50\mathrm{p}\mathrm{c}$ from the Galactic plane, suggests that the system has, at high likelihood, a significantly smaller center of mass velocity and a progenitor more massive than $2.1{\mathrm{M}}_{\odot }$ is ruled out (at 97% C.L.). A progenitor mass around $1.45{\mathrm{M}}_{\odot }$, involving a new previously unseen gravitational collapse, is kinematically favored. The low mass progenitor is consistent with the recent scintillation based velocity measurement of $66\pm 15\mathrm{k}\mathrm{m}/\mathrm{s}$ and rules out the high mass solution at 99% C.L. Conversely, if the unlikely higher mass solution is the true one we should increase the estimated rate of neutron star mergers by a factor of at least 2.