On the rapid orbital expansion in the compact low-mass X-ray binary 2A 1822$-$371

TL;DR

This paper proposes a magnetic wind-driven model to explain the rapid orbital expansion and high accretion rate observed in the low-mass X-ray binary 2A 1822$-$371, challenging standard magnetic braking theories.

Contribution

It introduces a new model involving strong magnetic fields and irradiation-driven winds to account for the system's peculiar orbital evolution and accretion behavior.

Findings

The model explains rapid orbital expansion in 2A 1822$-$371.

High accretion rates exceeding Eddington limit are accounted for.

Evolutionary tracks suggest magnetic wind effects are significant in such systems.

Abstract

The neutron star low-mass X-ray binary 2A 1822$-$371 has an orbital period of 5.57 hr. Mass transfer in such short-period binaries is thought to be driven by magnetic braking with orbital shrinking. However, 2A 1822$-$371 shows a very rapid orbital expansion, implying that mass transfer occurs rapidly in this system. The accretion rate of the neutron star is observationally estimated to be higher than the Eddington limit, which is also hard to be explained by the standard magnetic braking mechanism. In this work, we construct a model to account for the peculiar properties of 2A 1822$-$371. We assume that the donor star possesses a relatively strong magnetic field, which is coupled with the stellar winds excited by the X-ray radiation from the neutron star. This would generate efficient angular momentum loss, leading to a high mass transfer rate and hence orbital expansion. We provide possible evolutionary tracks of 2A 1822$-$371 and study how the input parameters affect the results. The observational implications of the irradiation-driven mass loss are also briefly discussed in the context of evolution of low-mass X-ray binaries and millisecond pulsars.