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.

Figure 1

The maximal kinematically allowed mass loss $\Delta m_{\max }$ and progenitor mass $m_{Bi}$ for a given change in the c.m. velocity $\Delta v_{\mathrm{c}.\mathrm{m}.}$. The upper mass depends on the actual eccentricity ($0.088<e<0.14$), hence the finite width of the line. The hatched area is kinematically excluded. The region allowed by standard evolutionary scenario through the formation of a He star, which requires a minimum mass of $2.1{\mathrm{M}}_{\odot }$ (marked $A$ [6]) to $2.3{\mathrm{M}}_{\odot }$ (marked $B$ [5]) to form a NS through a core collapse SN, is marked as shaded triangles. The no-kick solution that results with the above range of eccentricities is marked by the short heavy line.

Figure 2

(A) The probability that a binary system will end up within 50 pc from the Galactic plane, with $0.088<e<0.14$ and with a transverse velocity of $66\pm 15\mathrm{k}\mathrm{m}/\mathrm{s}$, given that 50 Myr before the progenitor system had a circular orbit, that star $\mathsf{B}$ had a progenitor mass $m_{Bi}$, and it obtained a random kick velocity of size $v_{\mathrm{k}\mathrm{i}\mathrm{c}\mathrm{k}}$. The probabilities are normalized to the most likely $M_{Bi},v_{\mathrm{k}\mathrm{i}\mathrm{c}\mathrm{k}}$. The He star solution requires $m_{Bi}$ larger than about $2.1{\mathrm{M}}_{\odot }$. With a fine-tuned $v_{\mathrm{k}\mathrm{i}\mathrm{c}\mathrm{k}}$, this type of a solution is ruled out at the 99% C.L. Solutions with either $m_{Bi}\approx (1.55\pm 0.2){\mathrm{M}}_{\odot }$ and $v_{\mathrm{k}\mathrm{i}\mathrm{c}\mathrm{k}}\lesssim 30\mathrm{k}\mathrm{m}/\mathrm{s}$, or $m_{Bi}\approx (1.45\pm 0.2){\mathrm{M}}_{\odot }$ and $v_{\mathrm{k}\mathrm{i}\mathrm{c}\mathrm{k}}\sim 50–80\mathrm{k}\mathrm{m}/\mathrm{s}$, are kinematically more favorable. In (B), the constraint on the transverse velocity is alleviated. Here, He star scenarios can be ruled out only at the $97\%$ C.L. The top left side is kinematically forbidden (no solution without a large $v_{\mathrm{k}\mathrm{i}\mathrm{c}\mathrm{k}}$ exists to counter the ejected mass), while the right-hand side is statistically disfavored since a large $v_{\mathrm{k}\mathrm{i}\mathrm{c}\mathrm{k}}$ implies a low probability for finding the system near the Galactic plane. (C) is the same as (A), except that we include the constraint that the system is seen almost edge on. (D) is the same as (A), except that here the transverse velocity is $141\pm 8.5\mathrm{k}\mathrm{m}/\mathrm{s}$. Note that, as marked on (B), while the region on the upper left side—high mass and low kick velocity—is kinematically forbidden, the region on the right-hand side and, in particular, the top right-hand side—high mass and large kick velocity—is allowed kinematically; it is just disfavored statistically.