Low Mass X-ray Binaries: The Effects of the Magnetic Braking Prescription

Abstract

We present a population study of low- and intermediate-mass X-ray binaries (LMXBs) with neutron star accretors, performed using the detailed 1D stellar evolution code MESA. We identify all plausible Roche-lobe overflowing binaries at the start of mass transfer, and compare our theoretical mass transfer tracks to the population of well-studied Milky Way LMXBs. The mass transferring evolution depends on the accepted magnetic braking (MB) law for angular momentum loss. The most common MB prescription ("Skumanich MB") originated from observations of the time-dependence of rotational braking of Sun-type stars, where the angular momentum loss rate depends on the donor mass $M_d$, donor radius $R_d$, and rotation rate $\Omega$, $\dot{J}\propto M_d{R}_d^{\gamma }{\Omega }^3$. The functional form of the Skumanich MB can be also obtained theoretically assuming a radial magnetic field, isotropic isothermal winds, and boosting of the magnetic field by rotation. Here we show that this simple form of the Skumanich MB law gives mass transfer rates an order of magnitude too weak to explain most observed persistent LMXBs. This failure suggests that the standard Skumanich MB law should not be employed to interpret Galactic, or extragalactic, LMXB populations, with either detailed stellar codes or rapid binary population synthesis codes. We investigate modifications for the MB law, and find that including a scaling of the magnetic field strength with the convective turnover time, and a scaling of MB with the wind mass loss rate, can reproduce persistent LMXBs, and does a better job at reproducing transient LMXBs.

20 pages, 10 figures, accepted to MNRAS

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Cited by in corpus (11)

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• Magnetic braking saturates: evidence from the orbital period distribution of low-mass detached eclipsing binaries from ZTF
• PSR J1012+5307: a millisecond pulsar with an extremely low-mass white dwarf companion
• Ultra-compact X-ray binaries with He star companions
• Crust structure and thermal evolution of neutron stars in soft X-ray transients
• Indications of non-conservative mass-transfer in AMXPs
• Convection and rotation boosted prescription of magnetic braking: application to the formation of extremely low-mass white dwarfs
• The peculiar chemical abundance of the transitional millisecond pulsar PSR J1023+0038 -- Li enhancement
• On the rapid orbital expansion in the compact low-mass X-ray binary 2A 1822$-$371