The Formation of Low‐Mass Transient X‐Ray Binaries

We consider constraints on the formation of low-mass X-ray binariescontaining neutron stars (NLMXBs) arising from the presence of soft X-raytransients among these systems. We show that in short-period systems driven byangular momentum loss these constraints require the secondary at the beginningof mass transfer to have a mass > 1.2 M_sun, and to be significantlynuclear-evolved. As a consequence a comparatively large fraction of suchsystems appear as soft X-ray transients even at short periods, as observed.Moreover the large initial secondary masses account for the rarity of NLMXBs atperiods less than 3 hr. In contrast, NLMXB populations forming with large kickvelocities would not have these properties, suggesting that the kick velocityis generally small compared to the pre-SN orbital velocity in a large fractionof systems. We derive constraints on progenitor system parameters and on thestrength of magnetic braking.Comment: Accepted for publication in ApJ, 19 pages, 4 figure