The Galactic Population of Low‐ and Intermediate‐Mass X‐Ray Binaries

Abstract

We present the first study that combines binary population synthesis in the Galactic disk and detailed evolutionary calculations of low-and intermediate-mass X-ray binaries (L/IMXBs). This approach allows us to follow completely the formation of L/IMXBs, and their evolution through the X-ray phase, to the point when they become binary millisecond pulsars (BMPs). We show that the formation probability of IMXBs with initial donor masses of 1.5-4 M ⊙ is typically 5 times higher than that of standard LMXBs with initial donor masses of <1.5 M ⊙ . Since IMXBs evolve to resemble observed LMXBs, we suggest that the majority of the observed systems may have descended from IMXBs. Distributions at the current epoch of the orbital periods, donor masses, and mass accretion rates of L/IMXBs have been computed, as have orbital-period distributions of BMPs. Several significant discrepancies between the theoretical and observed distributions are discussed. We find that the total number of luminous (L X > 10 36 ergs s −1 ) X-ray sources at the current epoch and the period distribution of BMPs are very sensitive to the parameters in analytic formula describing the common-envelope phase that precedes the formation of the neutron star. The orbital-period distribution of observed BMPs strongly favors cases where the common envelope is more easily ejected. However, this leads to a 100-fold overproduction of the theoretical number of luminous X-ray sources relative to the total observed number of LMXBs. X-ray irradiation of the donor star may result in a dramatic reduction in the X-ray active lifetime of L/IMXBs, thus possibly resolving the overproduction problem, as well as the long-standing BMP/LMXB birthrate problem.

On the nature of the break in the X-ray luminosity function of low-mass X-ray binaries

“…Belczynski et al 2002;Pfahl et al 2003;van der Sluys et al 2005;Fragos et al 2008), which has been affected by substantial uncertainties in the binary star evolution (e.g. the treatment of the common envelope stage, parameters of magnetic braking, etc.).…”

The MUCHFUSS project – searching for hot subdwarf binaries with massive unseen companions

“…Binary evolution theory Pfahl et al 2003) predicts the existence of sdB+NS/BH systems formed after two phases of unstable mass transfer and one supernova explosion. The predicted fraction of sdB+NS/BH systems ranges from about 1% to 2% of the close sdB binaries (Geier et al 2010b;Yungelson & Tutukov 2005;Nelemans 2010).…”

Accretion, ablation and propeller evolution in close millisecond pulsar binary systems

“…These estimates placed the LMXB binary birth rate at ∼ 10 −7 yr −1 assuming an observable X-ray lifetime of ∼ 10 9 yr (Pfahl, Rappaport & Podsiadlowski 2003;Hurley et al 2010).…”

“…This simple scenario leads to an energy balance equation of CE evolution, of which the uncertainties are encapsulated into two parameters, the CE efficiency parameter, α CE , and the stellar structure parameter, λ, related to the binding energy of the CE. Much uncertainty surrounds the evaluation of α CE (Tutukov & Yungelson 1996;Kiel & Hurley 2006), however, typically α CE = 1 (Willems & Kolb 2002;Pfahl et al 2003;Voss & Tauris 2003;Kiel & Hurley 2006), α CE λ = 1 (Belczynski et al 2002;Nelemans & Tout 2005;Pfahl, Podsiadlowski & Rappaport 2005;Belczynski et al 2008) or α CE = 3 (Kiel & Hurley 2006;Kiel et al 2008;Kiel, Hurley & Bailes 2010;Hurley et al 2010).…”