Born to be recycled: a comprehensive population synthesis of the Galactic millisecond pulsars

Abstract

Millisecond pulsars (MSPs) are the oldest but fastest pulsars known to date. In the 1980s, to explain how these pulsars could be formed, a new hypothesis was formulated: the recycling of pulsars, i.e the fact that a pulsar could accrete matter from a companion and been spun up. In this paper, we developed a population synthesis algorithm for pulsars which belong to a binary, in order to check whether most of the observed recycled pulsars were formed via an accretion mechanism and derive statistics about their properties, that are difficult to obtain through observations. We also make predictions for future surveys. Toward the presented objectives, we use the code Stellar EVolution for N-body (SEVN) to take into account all the binary processes and our own code to evolve each pulsar self-consistently by taking into account the secular evolution of a force-free magnetosphere, the magnetic field decay, gravitational braking and spatial evolution. Each pulsar is born in binary with a main sequence companion, and evolve to present time. The radio and $γ$-ray emission locations were modeled by the polar cap geometry and striped wind model, respectively. Our simulations seem to reproduce well the population of radio and $γ$-ray pulsars observed in the selected surveys. We also found that there should be less than $220$ unidentified pulsars in the Fourth Fermi-LAT catalogue of $γ$-ray sources (4FGL). High values of the viewing angle $ζ$ seem to be needed to be able to observe the recycled pulsars, and it also seems difficult to observe recycled pulsars with an aligned rotation axis and magnetic axis (i.e., $χ\leq 10$°). We find that only a small fraction, approximately $\sim 7.6\times10^{-3}$ %, of oxygen-neon white dwarfs (ONeWDs) in binary systems appear to contribute to the population of mildly recycled pulsars through accretion-induced collapse.