Magnetic Inclination Evolution of Accreting Neutron Stars in Intermediate/Low-Mass X-ray Binaries

TL;DR

This study models the long-term evolution of magnetic inclination angles in accreting neutron stars within binary systems, revealing dependencies on initial conditions and mass transfer history, with implications for various observed X-ray sources.

Contribution

It introduces a comprehensive simulation of magnetic inclination evolution in accreting neutron stars, considering binary evolution and mass transfer effects, which was not systematically studied before.

Findings

Magnetic inclination angle evolution depends on initial binary parameters.

Mass transfer history influences the magnetic inclination evolution.

Distribution of inclination angles varies among different X-ray binary types.

Abstract

The magnetic inclination angle $\chi$, namely the angle between the spin and magnetic axes of a neutron star (NS), plays a vital role in its observational characteristics. However, there are few systematic investigations on its long-term evolution, especially for accreting NSs in binary systems. Applying the model of \citet{2021MNRAS.505.1775B} and the binary evolution code \mesa{}, we simultaneously simulate the evolution of the accretion rate, spin period, magnetic field, and magnetic inclination angle of accreting NSs in intermediate/low X-ray binaries (I/LMXBs). We show that the evolution of $\chi$ depends not only on the initial parameters of the binary systems, but also on the mass transfer history and the efficiency of pulsar loss. Based on the calculated results we present the characteristic distribution of $\chi$ for various types of systems including ultracompact X-ray binaries, binary millisecond pulsars, and ultraluminous X-ray sources, and discuss their possible observational implications.