A strange star scenario for the formation of isolated millisecond pulsars

¹ Xinjiang Astronomical Observatory, CAS, Urumqi, Xinjiang 830011, PR China
e-mail: na.wang@xao.ac.cn
² School of Physics and Electrical Information, Shangqiu Normal University, Shangqiu, Henan 476000, PR China
e-mail: chenwc@pku.edu.cn
³ Key laboratory of Modern Astronomy and Astrophysics, Nanjing University, Ministry of Education, Nanjing 210046, PR China
e-mail: lixd@nju.edu.cn

Received: 25 January 2019
Accepted: 17 November 2019

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 ∼60 km s⁻¹ can produce ∼6 × 10³ isolated MSPs and birth rate of ∼6.6 × 10⁻⁷ 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.