A strange star scenario for the formation of isolated millisecond pulsars

TL;DR

This paper proposes a model where neutron stars in binaries collapse into strange stars, with asymmetric mass loss causing some to become solitary millisecond pulsars, aligning with observed solitary MSPs.

Contribution

It introduces a strange star formation scenario during the neutron star to strange star phase transition to explain solitary MSPs, supported by population-synthesis simulations.

Findings

Approximately 6,000 isolated MSPs produced by the model

Predicted birth rate of isolated MSPs matches observations

Mass and delay time distributions provided for future observational comparison

Abstract

According to the recycling model, neutron stars in low-mass X-ray binaries were spun up to millisecond pulsars (MSPs), which indicates that all MSPs in the Galactic plane ought to be harbored in binaries. However, about $20\%$ Galactic field MSPs are found to be solitary. To interpret this problem, we assume that the accreting neutron star in binaries may collapse and become a strange star when it reaches some critical mass limit. Mass loss and a weak kick induced by asymmetric collapse during the phase transition (PT) from neutron star to strange star can result in isolated MSPs. In this work, we use a population-synthesis code to examine the PT model. The simulated results show that a kick velocity of $\sim60~{\rm km~s}^{-1}$ can produce $\sim6\times10^3$ isolated MSPs and birth rate of $\sim6.6\times10^{-7} {\rm ~yr}^{-1}$ in the Galaxy, which is approximately in agreement with predictions from observations. For the purpose of comparisons with future observation, we also give the mass distributions of radio and X-ray binary MSPs, along with the delay time distribution.