Long-term evolution of spin and other properties of neutron star low-mass X-ray binaries: implications for millisecond X-ray pulsars

TL;DR

This study models the long-term evolution of neutron star low-mass X-ray binaries to understand their spin-up process and match observed millisecond pulsar parameters, highlighting the importance of detailed mass and accretion modeling.

Contribution

It provides a comprehensive computational framework for NS LMXB evolution using MESA, incorporating baryonic to gravitational mass conversion and exploring parameter effects on spin evolution.

Findings

NS spin frequency evolution depends on accretion rate and magnetic braking.

Model successfully matches observed parameters of four AMXPs.

Highlights challenges in explaining AMXPs with brown dwarf companions.

Abstract

A neutron star (NS) accreting matter from a companion star in a low-mass X-ray binary (LMXB) system can spin up to become a millisecond pulsar (MSP). Properties of many such MSP systems are known, which is excellent for probing fundamental aspects of NS physics when modelled using the theoretical computation of NS LMXB evolution. Here, we systematically compute the long-term evolution of NS, binary and companion parameters for NS LMXBs using the stellar evolution code MESA. We consider the baryonic to gravitational mass conversion to calculate the NS mass evolution and show its cruciality for the realistic computation of some parameters. With computations using many combinations of parameter values, we find the general nature of the complex NS spin frequency ($\nu$) evolution, which depends on various parameters, including accretion rate, fractional mass loss from the system, and companion star magnetic braking. Further, we utilize our results to precisely match some main observed parameters, such as $\nu$, orbital period ($P_{\rm orb}$), etc., of four accreting millisecond X-ray pulsars (AMXPs). By providing the $\nu$, $P_{\rm orb}$ and the companion mass spaces for NS LMXB evolution, we indicate the distribution and plausible evolution of a few other AMXPs. We also discuss the current challenges in explaining the parameters of AMXP sources with brown dwarf companions and indicate the importance of modelling the transient accretion in LMXBs as a possible solution.