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

TL;DR

This work develops a comprehensive binary population synthesis framework to test the accretion-driven recycling scenario for millisecond pulsars by evolving binaries with SEVN and by modelling NS spin, magnetic-field decay, and spatial motion under a force-free magnetosphere. It couples this evolution with explicit radio and gamma-ray detection prescriptions, including a striped-wind gamma-ray emission model, to reproduce the observed MSP populations and predict future survey yields. The results indicate most MSPs accrete for $\sim10^{8-9}$ yr, masses cluster around $1.8\,M_{\odot}$ (with some up to $\sim2.7\,M_{\odot}$), and roughly 80% have spin-orbit alignment $\alpha\lesssim 10^{\circ}$; the model also suggests $\sim190$ MSPs born in the Galactic field contribute about 5% of the GeV excess. The study finds that the striped-wind gamma-ray model can reproduce the observed gamma-ray peak-separation distributions, and it forecasts that future surveys (e.g., SKA) could detect roughly three times as many radio MSPs and five times as many gamma-ray MSPs, with implications for the origin of the GeV excess and the Galactic MSP census.

Abstract

Millisecond pulsars (MSPs) are the oldest but fastest pulsars known to date. 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 $330$ 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 $\sim 7.5\times10^{-3}$ %, of oxygen-neon white dwarfs (ONeWDs) in binary systems appear to contribute to the population of mildly recycled pulsars through accretion-induced collapse.

Figures (12)

• Figure 1: $P-\dot{P}$ diagram of the observed pulsars considered in this work along with the death line, green solid line, and death valley, shaded green area.
• Figure 2: Evolution of a pulsar in the simulation which becomes a MSP in the $P-\dot{P}$ diagram.
• Figure 3: Left panel: $P-\dot{P}$ diagram for the detected pulsars in radio surveys only in the simulations in red, along with the observations in radio for the FAST GPPS or PMPS survey in blue. Middle panel: $P-\dot{P}$ diagram for the detected pulsars in $\gamma$-ray surveys only in the simulations in green, along with the observed pulsars in $\gamma$-ray from Fermi surveys only in blue. Right panel: $P-\dot{P}$ diagram for the simulated pulsars detected both in radio and $\gamma$-ray surveys in purple, along with observations for the FAST GPPS or PMPS survey simultaneously with the $\gamma$-ray observations from Fermi in blue. For all the panels, the death line is represented as a green solid line, and the death valley is the shaded green area.
• Figure 4: Distribution of the $\gamma$-ray peak separation of the simulated population along with the observations from the 3PC catalogue.
• Figure 5: Width of the radio profile plotted against the spin period $P$ of the radio detected pulsars in the simulation along with the observations.