Double and single recycled pulsars: an evolutionary puzzle?

TL;DR

This study examines the discrepancy in observed numbers of isolated recycled pulsars versus double neutron star binaries, finding that lower natal kick velocities in binaries better explain the data, challenging standard models.

Contribution

It demonstrates through population synthesis that lower natal kick velocities are needed to match observations, suggesting neutron stars in binaries receive smaller kicks than single pulsars.

Findings

Standard models overpredict isolated recycled pulsars by a factor of several.

A natal kick velocity dispersion of 170 km/s aligns models with observations.

Binary pulsar detection bias is small (~25%) and insufficient to explain the discrepancy.

Abstract

We investigate the statistics of isolated recycled pulsars and double neutron star binaries in the Galactic disk. Since recycled pulsars are believed to form through accretion and spinup in close binaries, the isolated objects presumably originate from disrupted progenitors of double neutron stars. There are a comparable number of double neutron star systems compared to isolated recycled pulsars. We find that standard evolutionary models cannot explain this fact, predicting several times the number of isolated recycled pulsars than those in double neutron star systems. We demonstrate, through population synthesis calculations, that the velocity distribution of isolated recycled pulsars is broader than for binary systems. When this is accounted for in a model for radio pulsar survey selection effects, which include the effects of Doppler smearing for the double neutron star binaries, we find that there is a small (25%) bias towards the detection of double neutron star systems. This bias, however, is not significant enough to explain the observational discrepancy if standard (sigma = 265 km/s) neutron star natal kick velocities are invoked in binary population syntheses. Population syntheses in which the 1D Maxwellian velocity dispersion of the natal kick is sigma=170 km/s are consistent with the observations. These conclusions further support earlier findings the neutron stars formed in close interacting binaries receive significantly smaller natal kicks than the velocities of Galactic single pulsars would seem to indicate.