A New Class of High-Mass X-Ray Binaries: Implications for Core Collapse and Neutron Star Recoil

We investigate an interesting new class of high-mass X-ray binaries (HMXBs) with long orbital periods (Porb > 30 days) and low eccentricities (e ≲ 0.2). The orbital parameters suggest that the neutron stars in these systems did not receive a large impulse, or "kick," at the time of formation. After considering the statistical significance of these new binaries, we develop a self-consistent phenomenological picture wherein the neutron stars born in the observed wide HMXBs receive only a small kick (≲50 km s-1), while neutron stars born in isolation, in the majority of low-mass X-ray binaries, and in many of the well-known HMXBs with Porb ≲ 30 days receive the conventional large kicks, with a mean speed of ~300 km s-1. Assuming that this basic scenario is correct, we discuss a physical process that lends support to our hypothesis, whereby the magnitude of the natal kick to a neutron star born in a binary system depends on the rotation rate of its immediate progenitor following mass transfer—the core of the initially more massive star in the binary. Specifically, the model predicts that rapidly rotating precollapse cores produce neutron stars (NSs) with relatively small kicks, and vice versa for slowly rotating cores. If the envelope of the NS progenitor is removed before it has become deeply convective, then the exposed core is likely to be a rapid rotator. However, if the progenitor becomes highly evolved prior to mass transfer, then a strong magnetic torque, generated by differential rotation between the core and the convective envelope, may cause the core to spin down to the very slow rotation rate of the envelope. Our model has important implications for the dynamics of stellar core collapse, the retention of neutron stars in globular clusters, and the formation of double neutron star systems in the Galaxy.