Population synthesis for symbiotic X-ray binaries

Abstract

Symbiotic X-ray binaries (SyXBs) comprise a rare class of low-mass X-ray binaries. We study the Galactic SyXBs, which we consider as detached binaries composed of low-mass giants and wind-fed neutron star (NS) companions, by simulation of the interaction of a magnetized NS with its environment and utilizing a population synthesis code. We focus mainly on the parameters that influence the observational appearance of an SyXB: the donor wind velocity (vw) and the angular momentum distribution in the shell of matter settling on to an NS. We estimate the birthrate of SyXBs as ~4.1 × 10-5 to ~6.6 × 10-6yr-1 and their number in the Galaxy as ~100-1000. The assumed stellar wind velocity from cool giants is the input parameter that influences the model SyXB population most. Among known SyXBs or candidate systems, 4U 1954+31 and IGR J16358-4724 in which the NSs have very long spin periods may host quasi-spherically accreting NSs. GX 1+4 has a peculiar long-term spin behaviour and it may also be a quasi-spherical wind-accreting source. We cannot identify whether there are wind-fed accretion discs in 4U 1700+24, Sct X-1, IRXS J180431.1-273932 and 2XMM J174016.0-290337. © 2012 The Authors. Monthly Notices of the Royal Astronomical Society © 2012 RAS.