Donors of Persistent Neutron-Star Low-Mass X-Ray Binaries

Abstract

ABSTRACTProperties of X-ray luminosities in low-mass X-ray binaries (LMXBs) mainly depend on donors. We have carried out a detailed study of donors in persistent neutron-star LMXBs (PLMXBs) by means of a population synthesis code. PLMXBs with different donors have different formation channels. Our numerical simulations show that more than 90% of PLMXBs have main-sequence (MS) donors, and PLMXBs with red giant (RG) donors via stellar wind (wind) are negligible. In our model, most of the neutron stars (NSs) in PLMXBs with hydrogen-rich donors form via core-collapse supernovae, while more than 90% of the NSs in PLMXBs with naked helium star (He) donors or white dwarf (WD) donors form via an evolution-induced collapse via helium star (1.4 ≤ MHe/M⊙ ≤ 2.5) or an accretion-induced collapse for an accreting ONeMg WD. PLMXBs with different donors have different properties. In PLMXBs with MS donors, the orbital periods are between and 100 hr, and the mass transfer is driven by donor evolution or magnetic braking. Our population synthesis code shows that their X-ray luminosities are mainly around . Similarly, in PLMXBs with RG donors via Roche lobe overflow (Roche), the mass transfer is driven by donor evolution, but orbital periods are between and 1000 hr. Their X-ray luminosities are . The two known LMXBs (Cyg X-2 and GX 13 + 1) can belong to PLMXBs with RG (Roche) donors. PLMXBs with RG (wind) donors have the longest orbital periods and low X-ray luminosities (). Their contributions to X-ray luminosities can be negligible. In PLMXBs with He donors, the orbital periods are shorter than 80 minutes, and the mass transfer is mainly driven by magnetic braking. Results of our numerical simulations predict that PLMXBs with X-ray luminosities of mainly come from binaries with He donors. In PLMXBs with WD donors, the orbital periods are shorter than 1 hr, and the mass transfer is mainly driven by gravitational radiation. According to results of our population synthesis code, their X-ray luminosities are between ∼6 × 1035 and , and most of the LMXBs with WD donors are transient.