Kick Bimodality of Neutron Stars and Mode Dependence of Their Parameters

Why It Matters

The bimodal kick pattern reshapes theories of supernova asymmetry and neutron‑star birth, affecting predictions of pulsar demographics and related gravitational‑wave event rates.

Key Takeaways

• 45% of studied pulsars belong to low‑velocity natal kick mode.
• Magnetic fields <10⁹ G overrepresented among low‑velocity pulsars.
• No pulse‑width difference between low and high‑velocity groups.
• Age, distance, luminosity differences likely stem from selection bias.
• High‑velocity mode contains only one low‑field pulsar, an unexplained outlier.

Pulse Analysis

Supernova explosions are rarely perfectly symmetric; the resulting recoil—known as a natal kick—imparts velocities of hundreds of kilometres per second to newborn neutron stars. Decades of timing and proper‑motion measurements have hinted at a spread in kick magnitudes, but the precise shape of the distribution remained debated. Recent high‑precision astrometric data for isolated radio pulsars now support a clear bimodal structure, suggesting that distinct physical mechanisms may dominate in different progenitor scenarios, such as neutrino‑driven convection versus hydrodynamic instabilities.

The study’s statistical split shows that nearly half of the pulsars fall into a low‑velocity cohort, while the rest occupy a high‑velocity tail. Although age, distance and radio luminosity differ between the groups, these metrics are vulnerable to detection thresholds, explaining the observed bias. More compelling is the magnetic‑field divergence: low‑field (<10⁹ G) pulsars cluster in the low‑velocity mode, implying a possible link between magnetic‑field generation during core collapse and the magnitude of the kick. The solitary low‑field, high‑velocity outlier challenges existing models and may point to rare evolutionary pathways or measurement uncertainties.

Recognizing a bimodal kick distribution refines population‑synthesis simulations that forecast pulsar birthrates, Galactic distribution, and merger probabilities for double‑neutron‑star systems—key inputs for gravitational‑wave observatories. It also motivates theoretical work to reconcile magnetic‑field evolution with asymmetric explosion dynamics. Future surveys with the Square Kilometre Array and improved proper‑motion catalogs will test whether the observed split persists across larger samples, potentially unlocking a deeper understanding of the supernova engine.