Binary and neutron star evolution in low-mass X-ray binaries on the evolutionary tracks of accreting millisecond X-ray pulsars

TL;DR

This study models the binary and neutron star evolution in low-mass X-ray binaries to understand the formation and properties of accreting millisecond X-ray pulsars, aligning simulations with observational data.

Contribution

It introduces a comprehensive binary evolution analysis using MESA across different donor types, highlighting the initial orbital period's role and modeling neutron star spin evolution.

Findings

Initial orbital period significantly influences binary evolution.

Model reproduces typical properties of AMXPs.

Binary and neutron star properties are simultaneously matched in simulations.

Abstract

Neutron star low-mass X-ray binaries (LMXBs) are the progenitors of millisecond pulsars. In these systems, old neutron stars (NSs) can be spun up during a long-lasting accretion phase. The discovery of accreting millisecond X-ray pulsars (AMXPs) and transitional millisecond pulsars has provided key observational insights into the connection between millisecond pulsars and LMXBs. In this work, we have investigated both the binary system and the individual NS evolution leading to AMXP properties. We use MESA to analyse the binary evolution of LMXBs, following three distinct evolutionary tracks defined by the AMXP donor types. We find that while the magnetic braking index may affect the mass-transfer history, the initial orbital period is the most influential parameter that shapes the overall binary evolution. We use the mass accretion histories estimated from these binary simulations to study the rotational evolution of NSs employing the model that can account for torque-luminosity relations and the lack of X-ray pulses from most of these systems. With reasonable model parameters, our model results are in agreement with the typical properties of AMXPs. For these AMXP sources from each evolutionary track, we have shown that the model can reproduce the NS and binary properties simultaneously. Finally, we discuss the time-scales of different evolutionary paths, as well as the conditions under which these systems could be detectable at various stages of their evolution.