The Formation of Low‐Mass Transient X‐Ray Binaries

Abstract

We consider constraints on the formation of low-mass X-ray binaries containing neutron stars (NLMXBs) arising from the presence of soft X-ray transients among these systems. For a neutron star of mass M1 1.4 M☉ at formation, we show that in short-period (1-2 day) systems driven by angular momentum loss these constraints require the secondary at the beginning of mass transfer to have a mass of 1.3 M☉ M2 1.5 M☉ and to be significantly nuclear evolved, provided that supernova (SN) kick velocities are generally small compared with the pre-SN orbital velocity. As a consequence, a comparatively large fraction of such systems appear as soft X-ray transients even at short periods, as observed. Moreover, the large initial secondary masses account for the rarity of NLMXBs at periods P 3 hr. In contrast, NLMXB populations forming with large kick velocities would not have these properties, suggesting that the kick velocity is generally small compared with the pre-SN orbital velocity in a large fraction of systems, consistent with a recent reevaluation of pulsar proper motions. The results also place tight constraints on the strength of magnetic braking: if magnetic braking is significantly stronger than the standard form, too many unevolved NLMXBs would form; if it is slower by only a factor of 4, no short-period NLMXBs would form at all in the absence of a kick velocity. The narrow range for M2 found for negligible kick velocity implies restricted ranges near 4 M☉ for the helium star antecedent of the neutron star and near 18 M☉ for the original main-sequence progenitor. The pre-common-envelope period must lie near 4 yr, and we estimate the short-period NLMXB formation rate in the disk of the Galaxy as ~2 × 10-7 yr-1. Our results show that the neutron star mass at short-period NLMXB formation cannot be significantly larger than 1.4 M☉. Systems with formation masses of M1 1.2 M☉ would have disrupted, so observations implying M1 ~ 1.4 M☉ in some NLMXBs suggest that much of the transferred mass is lost from these systems.